0000822818-24-000008 | 10-K | XBRL Viewer

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BUSINESS COMBINATIONS - Proposed 2024 Acquisition (Details) $ in Millions	Feb. 02, 2024 USD ($)
HEPACO \| Subsequent Event
Business Acquisition [Line Items]
Purchase price to acquire business	$ 400.0

COMMITMENTS AND CONTINGENCIES

12 Months Ended

Dec. 31, 2023

Commitments and Contingencies Disclosure [Abstract]

COMMITMENTS AND CONTINGENCIES

The Company and its subsidiaries are subject to legal proceedings and claims arising in the ordinary course of business. Actions filed against the Company arise from commercial and employment-related claims including alleged class actions related to sales practices and wage and hour claims. The plaintiffs in these actions may be seeking damages or injunctive relief or both. These actions are in various jurisdictions and stages of proceedings, and some are covered in part by insurance. In addition, the Company’s waste management services operations are regulated by federal, state, provincial and local laws enacted to regulate discharge of materials into the environment, remediation of contaminated soil and groundwater or otherwise protect the environment. This ongoing regulation results in the Company frequently becoming a party to legal or administrative proceedings involving all levels of government authorities and other interested parties. The issues involved in such proceedings generally relate to alleged violations of existing permits and licenses or alleged responsibility under federal or state Superfund laws to remediate contamination at properties owned either by the Company or by other parties (“third-party sites”) to which either the Company or the prior owners of certain of the Company’s facilities shipped waste.

At December 31, 2023 and 2022, the Company had recorded reserves of $32.4 million and $37.1 million, respectively, for actual or probable liabilities related to the legal and administrative proceedings in which the Company was then involved, the principal of which are described below. As of December 31, 2023 and 2022, the $32.4 million and $37.1 million, respectively, of reserves consisted of (i) $25.0 million and $24.1 million, respectively, related to pending legal or administrative proceedings, including Superfund liabilities, which were included in remedial liabilities on the consolidated balance sheets, and (ii) $7.4 million and $13.0 million, respectively, primarily related to federal, state and provincial enforcement actions, which were included in accrued expenses on the consolidated balance sheets.

In management's opinion, it is not reasonably possible that the potential liability beyond what has been recorded, if any, that may result from these actions, either individually or collectively, will have a material effect on the Company's financial position, results of operations or cash flows. The Company periodically adjusts the aggregate amount of these reserves when actual or probable liabilities are paid or otherwise discharged, new claims arise, or additional relevant information about existing or probable claims becomes available.

Legal or Administrative Proceedings

As of December 31, 2023, the principal legal and administrative proceedings in which the Company was involved, or which had been terminated during 2023, relate to Safety-Kleen product liability cases and Superfund proceedings.

Safety-Kleen Product Liability Cases: Safety-Kleen, Inc. ("Safety-Kleen"), which is a legal entity acquired by the Company in 2012, has been named as a defendant in certain product liability cases that are currently pending in various courts and jurisdictions throughout the United States. As of December 31, 2023, there were approximately 70 proceedings (excluding cases which have been settled but not formally dismissed) wherein persons claim personal injury resulting from the use of Safety-Kleen's parts cleaning equipment or cleaning products. These proceedings typically involve allegations that the solvent used in Safety-Kleen's parts cleaning equipment contains contaminants and/or that Safety-Kleen's recycling process does not effectively remove the contaminants that become entrained in the solvent during their use. In addition, certain claimants assert that Safety-Kleen failed to warn adequately the product user of potential risks, including a historic failure to warn that solvent contains trace amounts of toxic or hazardous substances such as benzene.

The Company maintains insurance that it believes will provide coverage for these product liability claims (over amounts accrued for self-insured retentions and deductibles in certain limited cases), except for punitive damages to the extent not insurable under state law or excluded from insurance coverage. The Company historically has vigorously defended, and intends to continue to vigorously defend, itself and the safety of its products against all of these claims. Such matters are subject to many uncertainties and outcomes are not predictable with assurance. Consequently, the Company is unable to ascertain the ultimate aggregate amount of monetary liability or financial impact with respect to these matters as of December 31, 2023. From January 1, 2023 to December 31, 2023, 24 product liability claims were settled or dismissed. Due to the nature of these claims and the related insurance, the Company did not incur any expense as insurance provided coverage in full for all such claims. Safety-Kleen may be named in similar, additional lawsuits in the future, including claims for which insurance coverage may not be available.

Superfund Proceedings: The Company has been notified that either the Company (which, since December 28, 2012, has included Safety-Kleen) or the prior owners of certain of the Company's facilities for which the Company may have certain indemnification obligations have been identified as PRPs or potential PRPs in connection with 131 sites which are subject to or are proposed to become subject to proceedings under federal or state Superfund laws. Of the 131 Superfund related sites, six involve facilities that are now owned or leased by the Company and 125 involve third-party sites to which either the Company or the prior owners of certain of the Company’s facilities shipped waste. Of the 125 third-party sites, 30 are now settled, 13 are currently requiring expenditures on remediation and 82 are not currently requiring expenditures on remediation.

In connection with each site, the Company has estimated the extent, if any, to which it may be subject, either directly or as a result of any indemnification obligations, for cleanup and remediation costs, related legal and consulting costs associated with PRP investigations, settlements and related legal and administrative proceedings. The amount of such actual and potential liability is inherently difficult to estimate because of, among other relevant factors, uncertainties as to the legal liability (if any) of the Company or the prior owners of certain of the Company's facilities to contribute a portion of the cleanup costs, the assumptions that must be made in calculating the estimated cost and timing of remediation, the identification of other PRPs and their respective capability and obligation to contribute to remediation efforts and the existence and legal standing of indemnification agreements (if any) with prior owners, which may either benefit the Company or subject the Company to potential indemnification obligations. The Company believes its potential monetary liability could exceed $1.0 million at three of the 131 Superfund related sites.

Of the 125 third-party sites at which the Company has been notified it is a PRP or potential PRP or may have indemnification obligations, the Company has indemnification agreements at a total of 17 sites. These agreements indemnify the Company (which now includes Safety-Kleen) with respect to any liability at the 17 sites for waste disposed prior to the Company's (or Safety-Kleen's) acquisition of the former subsidiaries of Waste Management and McKesson which had shipped waste to those sites. Accordingly, the indemnifying parties are paying all costs of defending those subsidiaries in those 17 cases, including legal fees and settlement costs. However, there can be no guarantee that the Company's ultimate liabilities for those sites will not exceed the amount recorded or that indemnities applicable to any of these sites will be available to pay all or a portion of related costs. Except for those indemnification agreements discussed, the Company does not have an indemnity agreement with respect to any of the 125 third-party sites discussed above.

Federal, State and Provincial Enforcement Actions

From time to time, the Company pays fines or penalties in regulatory proceedings relating primarily to waste treatment, storage or disposal facilities. As of December 31, 2023, there were no proceedings for which the Company believes it is possible that the sanctions could equal or exceed $1.0 million. The Company believes that the fines or other penalties in this or any of the other regulatory proceedings will, individually or in the aggregate, not have a material effect on its financial condition, results of operations or cash flows.

Self-Insurance Liabilities

Under the Company's insurance programs, coverage is obtained for catastrophic exposures, as well as those risks required to be insured by law or contract. The Company's policy is to retain a significant portion of certain expected losses related to workers' compensation, employee medical, comprehensive general liability and vehicle liability. A portion of these self-insured liabilities are managed through its wholly-owned captive insurance subsidiary.

Provisions for losses expected under these programs are recorded based upon the Company's estimates of the aggregate liability for claims. The current deductible per participant per year for the employee medical insurance policy is $0.9 million. The current deductible per occurrence for workers' compensation is $1.0 million, general liability is $2.0 million and vehicle liability is $2.0 million. The retention per claim for the environmental impairment policy is $1.0 million. At December 31, 2023 and 2022, the Company had accrued $82.7 million and $74.9 million, respectively, for its self-insurance liabilities (exclusive of employee medical insurance) using a risk-free discount rate of 4.86% and 4.36%, respectively.

Anticipated payments for contingencies related to workers' compensation, comprehensive general liability and vehicle liability related claims at December 31, 2023 for each of the next five years and thereafter were as follows (in thousands):


Years ending December 31,
2024	$	33,954
2025	19,675
2026	14,663
2027	9,153
2028	6,275
Thereafter	9,650
Undiscounted self-insurance liabilities	93,370
Less: Discount	(10,670)
Total self-insurance liabilities (included in accrued expenses and other current liabilities)	$	82,700

OPERATIONS	12 Months Ended
OPERATIONS	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
OPERATIONS	OPERATIONS Clean Harbors, Inc., through its subsidiaries (collectively, the "Company"), is a leading provider of sustainable environmental and industrial services throughout North America. The Company is also the largest re-refiner and recycler of used oil and the premier provider of parts cleaning and related environmental services to commercial, industrial and automotive customers in North America.

SIGNIFICANT ACCOUNTING POLICIES - Advertising Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accounting Policies [Abstract]
Advertising expense	$ 10.9	$ 9.4	$ 6.0

Cover - USD ($)
$ in Billions

12 Months Ended

Dec. 31, 2023

Feb. 14, 2024

Jun. 30, 2023

Cover [Abstract]

Document Type

10-K

Document Annual Report

true

Document Period End Date

Dec. 31, 2023

Current Fiscal Year End Date

--12-31

Document Transition Report

false

Entity File Number

001-34223

Entity Registrant Name

CLEAN HARBORS, INC.

Entity Incorporation, State or Country Code

Entity Tax Identification Number

04-2997780

Entity Address, Address Line One

42 Longwater Drive

Entity Address, City or Town

Norwell

Entity Address, State or Province

Entity Address, Postal Zip Code

02061-9149

City Area Code

781

Local Phone Number

792-5000

Title of 12(b) Security

Common Stock, $0.01 par value

Trading Symbol

CLH

Security Exchange Name

NYSE

Entity Well-known Seasoned Issuer

Yes

Entity Voluntary Filers

Entity Current Reporting Status

Yes

Entity Interactive Data Current

Yes

Entity Filer Category

Large Accelerated Filer

Entity Small Business

false

Entity Emerging Growth Company

false

ICFR Auditor Attestation Flag

true

Document Financial Statement Error Correction

false

Entity Shell Company

false

Entity Public Float

$ 8.3

Entity Common Stock, Shares Outstanding

53,940,747

Documents Incorporated by Reference

DOCUMENTS INCORPORATED BY REFERENCE

Document

Incorporated as to

Proxy Statement for the

2024 Annual Meeting of Stockholders

Part III

Entity Central Index Key

0000822818

Document Fiscal Year Focus

2023

Document Fiscal Period Focus

Amendment Flag

false

FINANCING ARRANGEMENTS - Summary of Redemption Prices (Details) - Unsecured debt	12 Months Ended
	Dec. 31, 2023
Debt Instrument, Redemption [Line Items]
Redemption percentage of principal amount	100.00%
2031 Notes
Debt Instrument, Redemption [Line Items]
Redemption percentage of principal amount	106.375%
Debt Instrument, Redemption, Period One \| 2029 Notes
Debt Instrument, Redemption [Line Items]
Redemption percentage of principal amount	102.563%
Debt Instrument, Redemption, Period One \| 2031 Notes
Debt Instrument, Redemption [Line Items]
Redemption percentage of principal amount	103.188%
Debt Instrument, Redemption, Period Two \| 2027 Notes
Debt Instrument, Redemption [Line Items]
Redemption percentage of principal amount	101.219%
Debt Instrument, Redemption, Period Two \| 2029 Notes
Debt Instrument, Redemption [Line Items]
Redemption percentage of principal amount	101.281%
Debt Instrument, Redemption, Period Two \| 2031 Notes
Debt Instrument, Redemption [Line Items]
Redemption percentage of principal amount	101.594%
Debt Instrument, Redemption, Period Three \| 2027 Notes
Debt Instrument, Redemption [Line Items]
Redemption percentage of principal amount	100.00%
Debt Instrument, Redemption, Period Three \| 2029 Notes
Debt Instrument, Redemption [Line Items]
Redemption percentage of principal amount	100.00%
Debt Instrument, Redemption, Period Three \| 2031 Notes
Debt Instrument, Redemption [Line Items]
Redemption percentage of principal amount	100.00%

REMEDIAL LIABILITIES (Tables)

12 Months Ended

Dec. 31, 2023

Environmental Remediation Obligations [Abstract]

Schedule of Changes to Remedial Liabilities

The changes to remedial liabilities from January 1, 2022 through December 31, 2023 were as follows (in thousands):


	Remedial Liabilities for Landfill Sites		Remedial Liabilities for Inactive Sites		Remedial Liabilities (Including Superfund) for Non-Landfill Operations		Total
Balance at January 1, 2022	$	1,780	$	59,787	$	50,306	$	111,873
Liabilities assumed in acquisition of real estate	—		—		8,092		8,092
Measurement period adjustments from prior period acquisition	—		—		338		338
Accretion	86		2,498		1,607		4,191
Changes in estimates recorded to consolidated statement of operations	8		877		612		1,497
Environmental expenditures	(50)		(3,370)		(5,375)		(8,795)
Currency translation and other	—		(43)		(863)		(906)
Balance at December 31, 2022	1,824		59,749		54,717		116,290


Accretion	89		2,540		1,561		4,190
Changes in estimates recorded to consolidated statement of operations	19		1,713		1,896		3,628
Expenditures	(52)		(3,840)		(9,384)		(13,276)
Currency translation and other	—		115		296		411
Balance at December 31, 2023	$	1,880	$	60,277	$	49,086	$	111,243

Schedule of Remedial Liabilities Anticipated Payments for Each of the Next Five Years

Anticipated payments at December 31, 2023 (based on current estimated costs and anticipated timing of necessary regulatory approvals to commence work on remedial activities) for each of the next five years and thereafter were as follows (in thousands):


Years ending December 31,
2024	$	13,898
2025	19,253
2026	9,006
2027	7,001
2028	7,767
Thereafter	71,737
Undiscounted remedial liabilities	128,662
Less: Discount at risk free rates	(17,419)
Total remedial liabilities	$	111,243

Schedule of Environmental Exit Costs by Cost

The following table presents the Company's estimated remedial liabilities and reasonably possible additional liabilities as of December 31, 2023 disaggregated by facility/site type (in thousands, except percentages):


Type of Facility or Site	Remedial Liabilities		% of Total		Reasonably Possible Additional Liabilities (1)
Inactive facilities not used in active conduct of the Company's business, most of which were assumed through prior period acquisitions (24 facilities)	$	60,277	54.2	%	$	11,585
Facilities now used in active conduct of the Company's business (45 facilities)	43,299		38.9		9,080
Superfund sites (13 sites)	7,667		6.9		1,150
Total	$	111,243	100.0	%	$	21,815

___________________________________

(1)Amounts represent the high end of the range of management's best estimate of the reasonably possible additional liabilities.

The following table presents the Company's estimated remedial liabilities and reasonably possible additional liabilities as of December 31, 2023 disaggregated by facilities/sites which represent at least 5% of the total and with all other facilities/ sites combined (in thousands, except percentages):


Location	Type of Facility or Site	Remedial Liabilities(1)		% of Total		Reasonably Possible Additional Liabilities(2)
Baton Rouge, LA	Closed incinerator and landfill	$	27,883	25.1	%	$	4,868
Bridgeport, NJ	Closed incinerator	17,472		15.7		3,288
Mercier, Quebec	Idled incinerator	11,501		10.3		1,735
Linden, NJ	Operating solvent recycling center	9,614		8.6		2,187
Various	All other incinerators, landfills, wastewater treatment facilities and service centers (65 facilities)	37,106		33.4		8,587
Various	Superfund sites (each representing less than 5% of total liabilities) (13 sites)	7,667		6.9		1,150
Total		$	111,243	100.0	%	$	21,815

_________________________________

(1)$25.0 million of the $111.2 million remedial liabilities include estimates related to the legal and administrative proceedings discussed in Note 18, "Commitments and Contingencies."

(2)Amounts represent the high end of the range of management's best estimate of the reasonably possible additional liabilities.

SIGNIFICANT ACCOUNTING POLICIES - Defined Contribution Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accounting Policies [Abstract]
Employer discretionary contribution	$ 28.8	$ 25.3	$ 20.5

REMEDIAL LIABILITIES - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Site Contingency [Line Items]
Liabilities assumed in acquisition of real estate		$ 8.1
Remedial Liabilities for Inactive Sites
Site Contingency [Line Items]
Increase of remedial liabilities due to changes in estimates of related liabilities to account for new information obtained	$ 1.1
Remedial Liabilities For Active Sites
Site Contingency [Line Items]
Increase of remedial liabilities due to changes in estimates of related liabilities to account for new information obtained	$ 1.1

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References + Details

BUSINESS COMBINATIONS - 2023 Acquisitions (Details) - Thompson Industrial $ in Millions	Mar. 31, 2023 USD ($)
Business Acquisition [Line Items]
Purchase price to acquire business	$ 110.9
Weighted average useful life of acquired intangible assets (in years)	13 years
Minimum
Business Acquisition [Line Items]
Finite-lived intangible asset, useful life (in years)	5 years
Maximum
Business Acquisition [Line Items]
Finite-lived intangible asset, useful life (in years)	15 years

CLOSURE AND POST-CLOSURE LIABILITIES (Tables)

12 Months Ended

Dec. 31, 2023

Asset Retirement Obligation Disclosure [Abstract]

Schedule of Closure and Post-Closure Liabilities

The changes to closure and post-closure liabilities (also referred to as "asset retirement obligations") from January 1, 2022 through December 31, 2023 were as follows (in thousands):


	Landfill Retirement Liability		Non-Landfill Retirement Liability		Total
Balance at January 1, 2022	$	53,425	$	45,678	$	99,103
Liabilities assumed in acquisitions	—		55		55
Measurement period adjustments from prior period acquisitions	—		1,148		1,148
New asset retirement obligations	3,743		—		3,743
Accretion	4,605		4,147		8,752
Changes in estimates recorded to consolidated statement of operations	1,063		5,712		6,775
Changes in estimates recorded to consolidated balance sheet	3,219		1,742		4,961
Environmental expenditures	(3,373)		(1,778)		(5,151)
Currency translation and other	(431)		(154)		(585)
Balance at December 31, 2022	62,251		56,550		118,801
Liabilities assumed in acquisitions	—		574		574
Measurement period adjustments from prior period acquisitions	—		3,015		3,015
New asset retirement obligations	3,092		—		3,092
Accretion	4,958		4,519		9,477
Changes in estimates recorded to consolidated statement of operations	1,147		53		1,200
Changes in estimates recorded to consolidated balance sheet	(3,706)		1,600		(2,106)
Environmental expenditures	(8,478)		(7,206)		(15,684)
Currency translation and other	179		52		231
Balance at December 31, 2023	$	59,443	$	59,157	$	118,600

Schedule of Expected Payments Related to Asset Retirement Obligations

Anticipated payments (based on current estimated costs and anticipated timing of necessary regulatory approvals to commence work on closure and post-closure activities) for each of the next five years and thereafter are as follows (in thousands):


Years ending December 31,
2024	$	15,698
2025	16,694
2026	14,750
2027	3,955
2028	13,628
Thereafter	252,110
Undiscounted closure and post-closure liabilities	316,835
Less: Discount at credit-adjusted risk-free rate	(114,325)
Less: Undiscounted estimated closure and post-closure liabilities relating to airspace not yet consumed	(83,910)
Present value of closure and post-closure liabilities	$	118,600

COMMITMENTS AND CONTINGENCIES - Self Insurance Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Product Liability Contingency [Line Items]
Retention for environmental impairment	$ 1,000
Self-insurance liabilities	$ 82,700	$ 74,900
Weighted average risk free discount rate for self insurance liabilities	4.86%	4.36%
Self Insurance Losses Expected [Abstract]
2024	$ 33,954
2025	19,675
2026	14,663
2027	9,153
2028	6,275
Thereafter	9,650
Undiscounted self-insurance liabilities	93,370
Less: Discount	(10,670)
Total self-insurance liabilities (included in accrued expenses and other current liabilities)	82,700
Minimum
Product Liability Contingency [Line Items]
Deductible medical insurance policy	900
Safety-Kleen Sustainability Solutions
Product Liability Contingency [Line Items]
Deductible per occurrence for workers compensation	1,000
Deductible per occurrence for general liability	2,000
Deductible per occurrence for vehicle liability	$ 2,000

BUSINESS COMBINATIONS - 2022 Acquisitions (Details) - USD ($) $ in Thousands	Dec. 09, 2022	Jun. 17, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Business Acquisition [Line Items]
Goodwill			$ 1,287,736	$ 1,246,878	$ 1,227,042
June 17, 2022 Acquisition
Business Acquisition [Line Items]
Purchase price to acquire business		$ 78,900
Weighted average useful life of acquired intangible assets (in years)		18 years
Goodwill			$ 32,212	$ 32,015
June 17, 2022 Acquisition \| Minimum
Business Acquisition [Line Items]
Finite-lived intangible asset, useful life (in years)		5 years
June 17, 2022 Acquisition \| Maximum
Business Acquisition [Line Items]
Finite-lived intangible asset, useful life (in years)		20 years
December 9, 2022 Acquisition
Business Acquisition [Line Items]
Purchase price to acquire business	$ 12,600
Goodwill	$ 2,700

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References + Details

REVENUES

12 Months Ended

Dec. 31, 2023

Revenue from Contract with Customer [Abstract]

REVENUES

The Company generates revenues through the following operating segments: Environmental Services and Safety-Kleen Sustainability Solutions ("SKSS"). The Company's Environmental Services operating segment generally has four sources of revenue and the SKSS operating segment has two sources of revenue. The Company disaggregates third-party revenues by geographic location and source of revenue as management believes these categories depict how revenue and cash flows are affected by economic factors. The Company's significant sources of revenue include:

Technical Services—Technical Services contribute to the revenues of the Environmental Services operating segment. Revenues for these services are generated from fees charged for waste material management and disposal services including onsite environmental management services, collection and transportation, packaging, recycling, treatment and disposal of waste and remediation projects. These services handle hazardous and/or non-hazardous waste, including per- and polyfluoroalkyl substances ("PFAS"). Revenue is primarily generated by short-term projects, most of which are governed by master service agreements that are long-term in nature. These master service agreements are typically entered into with the Company's larger customers and outline the pricing and legal frameworks for such arrangements. Services are provided based on purchase orders or agreements with the customer and include prices based upon units of volume of waste, material and personnel costs as well as transportation and other fees. Collection and transportation revenues are recognized over time, as the customer receives and consumes the benefits of the services as they are being performed and the Company has a right to payment for performance completed to date. The Company uses the input method to recognize revenue over time, based on time and materials incurred as a basis for measuring the satisfaction of the performance obligation. Revenues for treatment and disposal of waste are recognized upon completion of treatment, final disposition in a landfill or incinerator, or when the waste is shipped to a third-party for processing and disposal. The Company periodically enters into bundled arrangements for the collection and transportation and disposal of waste. For such arrangements, transportation and disposal are considered distinct performance obligations and the Company allocates revenue to each based on the relative standalone selling price (i.e., the estimated price that a customer would pay for the services on a standalone basis). Revenues and the related costs from waste that is not yet completely processed and disposed of are deferred. The deferred revenues and costs are recognized when the services are completed. The period between collection and transportation and the final processing and disposal ranges depending on the location of the customer, but generally is measured in days.

Industrial Services—Industrial Services contribute to the revenues of the Environmental Services operating segment. These revenues are primarily generated from industrial and specialty services provided to refineries, chemical plants, manufacturing facilities, power generation companies and other industrial customers throughout North America. Services include in-plant cleaning and maintenance services, plant outage and turnaround services, specialty cleaning services including chemical cleaning, pigging and high and ultra-high pressure water cleaning, leak detection and repair, daylighting, production services and upstream energy services. Services are provided based on purchase orders or agreements with the customer and include prices based upon daily, hourly or job rates for equipment, materials and personnel. The Company recognizes revenue for these services over time, as the customer receives and consumes the benefits of the services as they are being performed and the Company has a right to payment for performance completed to date. The Company uses the input method to recognize revenue over time, based on time and materials incurred.

Field and Emergency Response Services—Field and Emergency Response Services contribute to the revenues of the Environmental Services operating segment. Field Services revenues are generated from cleanup services at customer sites, including those managed by municipalities and utility providers, or other locations on a scheduled or emergency response basis. Services include confined space entry for tank cleaning, site decontamination, remediation, railcar cleaning, manhole/vault clean outs, product recovery and transfer and vacuum services. Additional services include filtration and water treatment services. Response services for environmental emergencies of any scale range from man-made disasters such as oil spills to natural disasters like hurricanes. Emergency response services also include spill cleanup on land and water, as well as contagion disinfection, decontamination and disposal services. Field and emergency response services are provided based on purchase orders or agreements with customers and include prices generally based upon daily, hourly or job rates for equipment, materials and personnel. The Company recognizes revenue for these services over time, as the customer receives and consumes the benefits of the services as they are being performed and the Company has a right to payment for performance completed to date. The Company uses the input method to recognize revenue over time, based on time and materials incurred. The duration of such services can be over a number of hours, several days or even months for larger scale projects.

Safety-Kleen Environmental Services—Safety-Kleen Environmental Services revenues contribute both to the Environmental Services operating segment and the SKSS operating segment depending upon the nature of such revenues and operating responsibilities relative to executing the revenue contracts. Revenues from providing containerized waste handling and disposal services, parts washer services and vacuum services, referred to collectively as the Safety-Kleen branches' core service offerings, contribute to the revenues of the Environmental Services operating segment. In addition, sales of packaged blended oil products and other complementary product sales contribute to the revenues of the Environmental Services operating segment. Revenues generated from waste oil, anti-freeze and oil filter collection services, sales of bulk blended oil products and sales of bulk automotive fluids contribute to the SKSS operating segment.

Generally, the revenue from services is recognized over time, as the customer receives and consumes the benefits of the services as they are being performed and the Company has a right to payment for performance completed to date. The duration of such services can be over a number of hours or several days. The Company uses the input method to recognize revenue over time, based on time and materials incurred. Product revenue is recognized upon the transfer of control whereby control transfers when the products are delivered to the customer. Containerized waste services consist of profiling, collecting, transporting and recycling or disposing of a wide variety of waste. Related collection and transportation revenues are recognized over time, as the customer receives and consumes the benefits of the services as they are being performed and the Company has a right to payment for performance completed to date. Parts washer services include customer use of the Company's parts washer equipment, cleaning and maintenance of the parts washer equipment and removal and replacement of used cleaning fluids. Parts washer services are considered a single performance obligation due to the highly integrated and interdependent nature of the arrangement. Revenue from parts washer services is recognized over the service interval as the customer receives the benefit of the services.

Safety-Kleen Oil—Safety-Kleen Oil related sales contribute to the revenues of the SKSS segment. These revenues are generated from sales of high-quality base and blended lubricating oils to third-party distributors, government agencies, fleets, railroads and industrial customers. The business also sells recycled fuel oil to asphalt plants, industrial plants and pulp and paper companies. The used oil is also processed into vacuum gas oil which can be further re-refined into lubricant base oils or sold directly into the marine diesel oil fuel market. Revenue for oil products is recognized at a point in time, upon the transfer of control. Control transfers when the products are delivered to the customer.

The following tables present the Company's third-party revenue disaggregated by source of revenue and geography (in thousands):


	For the year ended December 31, 2023
	Environmental Services		Safety-Kleen Sustainability Solutions		Corporate		Total
Primary Geographical Markets
United States	$	4,022,394	$	846,339	$	447	$	4,869,180
Canada	447,515		92,457		—		539,972
Total third-party revenues	$	4,469,909	$	938,796	$	447	$	5,409,152

Sources of Revenue
Technical Services	$	1,563,847	$	—	$	—	$	1,563,847
Industrial Services and Other	1,418,938		—		447		1,419,385
Field and Emergency Response Services	609,913		—		—		609,913
Safety-Kleen Environmental Services	877,211		224,830		—		1,102,041
Safety-Kleen Oil	—		713,966		—		713,966
Total third-party revenues	$	4,469,909	$	938,796	$	447	$	5,409,152


	For the year ended December 31, 2022
	Environmental Services		Safety-Kleen Sustainability Solutions		Corporate		Total
Primary Geographical Markets
United States	$	3,675,880	$	899,780	$	507	$	4,576,167
Canada	469,093		121,345		—		590,438
Total third-party revenues	$	4,144,973	$	1,021,125	$	507	$	5,166,605

Sources of Revenue
Technical Services	$	1,495,264	$	—	$	—	$	1,495,264
Industrial Services and Other	1,311,072		—		507		1,311,579
Field and Emergency Response Services	575,300		—		—		575,300
Safety-Kleen Environmental Services	763,337		199,360		—		962,697
Safety-Kleen Oil	—		821,765		—		821,765
Total third-party revenues	$	4,144,973	$	1,021,125	$	507	$	5,166,605


	For the year ended December 31, 2021
	Environmental Services		Safety-Kleen Sustainability Solutions		Corporate		Total
Primary Geographical Markets
United States	$	2,631,112	$	693,542	$	299	$	3,324,953
Canada	394,795		85,818		—		480,613
Total third-party revenues	$	3,025,907	$	779,360	$	299	$	3,805,566

Sources of Revenue
Technical Services	$	1,209,624	$	—	$	—	$	1,209,624
Industrial Services and Other	705,999		—		299		706,298
Field and Emergency Response Services	466,380		—		—		466,380
Safety-Kleen Environmental Services	643,904		161,587		—		805,491
Safety-Kleen Oil	—		617,773		—		617,773
Total third-party revenues	$	3,025,907	$	779,360	$	299	$	3,805,566

Contract Balances


(in thousands)	December 31, 2023		December 31, 2022
Receivables	$	983,111	$	964,603
Contract assets (unbilled receivables)	107,859		107,010
Contract liabilities (deferred revenue)	95,230		94,094

The timing of revenue recognition, billings and cash collections results in billed accounts receivable, unbilled receivables (contract assets) and customer advances and deposits or deferred revenue (contract liabilities) on the consolidated balance sheet. Generally, billing occurs subsequent to revenue recognition, as a right to payment is not just subject to passage of time, resulting in contract assets, which are generally classified as current. The Company sometimes receives advances or deposits from its customers before revenue is recognized, resulting in contract liabilities. These assets and liabilities are reported on the consolidated balance sheet on a contract-by-contract basis at the end of each reporting period. The contract liability balances at the beginning of each period presented were generally fully recognized in the subsequent three-month period.

Variable Consideration

The nature of the Company's contracts give rise to certain types of variable consideration, including in limited cases volume discounts. Accordingly, management establishes a revenue allowance to cover the estimated amounts of revenue that may need to be credited to customers' accounts in future periods. The Company estimates the amount of variable consideration included in the estimated transaction price based on historical experience, anticipated performance and management's best judgment at the time. The Company includes estimated amounts in the transaction price to the extent it is probable that a significant reversal of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is resolved. There have been no material changes in estimates of variable consideration in the periods presented.

Contract Costs

Contract costs include direct and incremental costs to obtain or fulfill a contract. Parts washer costs include costs of solvent, commissions paid relating to revenue generated from parts washer services, and transportation costs associated with transferring the product picked up from the service as it is brought to the Company’s facilities or third-party sites. Costs related to the treatment of waste include costs for waste receiving, drum movement and storage, waste consolidation and transportation between facilities.

The Company’s contract costs that are subject to capitalization are comprised of costs associated with parts washer services and with the treatment and disposal of waste. As of December 31, 2023 and 2022, the Company's deferred contract costs totaled $29.9 million and $29.7 million, respectively. Deferred parts washer costs are recognized over the service interval as the customer receives the benefit of the services, and deferred costs related to treatment and disposal of waste are recognized when the corresponding waste is disposed. Deferred costs are included within total current assets in the Company’s consolidated balance sheets. The deferred contract cost balances at the beginning of each period presented were fully recognized in cost of revenue in the subsequent three-month period.

- Definition

The entire disclosure of revenue from contract with customer to transfer good or service and to transfer nonfinancial asset. Includes, but is not limited to, disaggregation of revenue, credit loss recognized from contract with customer, judgment and change in judgment related to contract with customer, and asset recognized from cost incurred to obtain or fulfill contract with customer. Excludes insurance and lease contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-9

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-10

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 15
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-15

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-12

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-12

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-12

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-12

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-12

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-13

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Topic 606
-Publisher FASB
-URI https://asc.fasb.org//606/tableOfContent

+ Details

PROPERTY, PLANT AND EQUIPMENT - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 4,964,301	$ 4,466,429
Less - accumulated depreciation and amortization	2,770,983	2,486,127
Total property, plant and equipment, net	2,193,318	1,980,302
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	174,891	172,579
Asset retirement costs (non-landfill)
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	27,167	22,001
Landfill assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	253,180	232,872
Buildings and improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	630,525	591,397
Right-of-use assets, finance leases	8,000	8,000
Vehicles
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,276,567	1,112,188
Right-of-use assets, finance leases	151,700	106,700
Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,388,370	2,195,064
Right-of-use assets, finance leases	9,200	9,200
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 213,601	$ 140,328

INVENTORIES AND SUPPLIES

12 Months Ended

Dec. 31, 2023

Inventory Disclosure [Abstract]

INVENTORIES AND SUPPLIES

Inventories and supplies consisted of the following (in thousands):


	December 31, 2023		December 31, 2022
Oil and oil related products	$	118,600	$	151,519
Supplies	177,217		143,743
Solvent and solutions	11,795		11,994
Other	19,899		17,738
Total inventories and supplies	$	327,511	$	324,994

The balance of oil and oil related products has decreased due to lower volumes of base and blended oil on hand at December 31, 2023 when compared with December 31, 2022 and overall price decreases for the oil inventory and additives. Supplies inventories consist primarily of critical spare parts to support the Company's incinerator and re-refinery operations and other general supplies used in our normal day-to-day operations. Other inventories consist primarily of parts washer components, cleaning fluids, absorbents and automotive fluids, such as windshield washer fluid and antifreeze.

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (PARENTHETICAL) - $ / shares	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Stockholders' Equity [Abstract]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01

INCOME TAXES

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

INCOME TAXES

The domestic and foreign components of income before provision for income taxes were as follows (in thousands):


	For the years ended December 31,
	2023		2022		2021
Domestic	$	401,912	$	406,206	$	223,438
Foreign	101,367		131,792		46,277
Total	$	503,279	$	537,998	$	269,715

The provision for income taxes consisted of the following (in thousands, except percentages):


	For the years ended December 31,
	2023		2022		2021
Current:
Federal	$	64,164	$	52,237	$	42,480
State	25,496		26,980		18,126
Foreign	23,078		29,488		4,380
	112,738		108,705		64,986
Deferred
Federal	18,251		32,199		2,275
State	(9,049)		(2,432)		(4,777)
Foreign	3,483		(12,218)		3,984
	12,685		17,549		1,482
Provision for income taxes	$	125,423	$	126,254	$	66,468

Effective tax rate	24.9%		23.5%		24.6%

The Company's effective income tax rate varied from the amount computed using the statutory federal income tax rate of 21% as follows (in thousands):


	For the years ended December 31,
	2023		2022		2021
Tax expense at U.S. statutory rate	$	105,689	$	112,980	$	56,640
State income taxes, net of federal benefit	18,067		19,831		12,101
Foreign rate differential	4,213		6,196		1,922
Valuation allowance	(7,699)		(18,769)		(9,139)
Uncertain tax position interest and penalties	(7)		(2,454)		263
Tax credits expired	1,653		2,768		2,530
Non-deductible compensation	2,898		2,754		2,326
Other	609		2,948		(175)
Provision for income taxes	$	125,423	$	126,254	$	66,468

The valuation allowance benefits recognized in 2023 are predominantly the result of cumulative profitable earnings at certain US entities with historic State operating losses. As of December 31, 2023, management determined that there is sufficient positive evidence to conclude that it is more likely than not that additional deferred taxes associated with these State net operating losses are realizable. Therefore, the valuation allowance was reduced accordingly.

The valuation allowance benefits recognized in 2022 were the result of cumulative profitable earnings at certain Canadian entities with historic operating losses. The 2022 earnings were sufficient to utilize substantially all of the net operating losses, and as of December 31, 2022, the Company released any remaining valuation allowances associated with these entities, which were nominal.

The valuation allowance benefits recognized in 2021 were predominately related to taxable earnings in certain Canadian entities that benefited from amounts received under the Canada Emergency Wage Subsidiary. In addition, foreign tax credits

that expired in the period presented had full valuation allowances which were also written off, contributing to the valuation allowance benefits in the table above. The foreign tax credit expirations and associated valuation allowance write-offs had no net impact to the provision for income taxes in any year.

The components of the total net deferred tax assets and liabilities as of December 31, 2023 and 2022 were as follows (in thousands):


	2023		2022
Deferred tax assets:

Provision for doubtful accounts	$	10,882	$	11,544
Closure, post-closure and remedial liabilities	31,944		31,837
Operating lease liabilities	46,784		42,255
Accrued expenses	14,963		19,311
Accrued compensation and benefits	15,058		20,171
Net operating loss carryforwards(1)	39,042		41,585
Tax credit carryforwards(2)	6,531		8,903

Stock-based compensation	3,516		3,988
Other	4,843		7,487
Total deferred tax assets	173,563		187,081
Deferred tax liabilities:
Property, plant and equipment	(284,997)		(281,131)
Operating lease right-of-use assets	(46,584)		(41,939)
Interest rate swap asset	(9,576)		(17,587)
Permits and other intangible assets	(130,391)		(132,681)
Prepaid expenses	(12,372)		(12,088)
Total deferred tax liabilities	(483,920)		(485,426)
Total net deferred tax liability before valuation allowance	(310,357)		(298,345)
Less valuation allowance	(35,272)		(42,509)
Net deferred tax liabilities	$	(345,629)	$	(340,854)

___________________________________

(1)As of December 31, 2023, the net operating loss carryforwards included (i) state net operating loss carryforwards of $229.9 million which will begin to expire in 2024, (ii) federal net operating loss carryforwards of $26.6 million which will begin to expire in 2024 and (iii) foreign net operating loss carryforwards of $83.2 million which will begin to expire in 2024.

(2)As of December 31, 2023, the foreign tax credit carryforwards of $4.4 million will begin to expire in 2024.

The Company previously recognized the U.S. federal income taxes related to the operations in Canada and has not accrued for any remaining undistributed foreign earnings. These amounts continue to be indefinitely reinvested. The amount of tax associated with those unrepatriated earnings is not expected to be material.

A valuation allowance is required to be established when, based on an evaluation of available evidence, it is more likely than not that some portion or all of the deferred tax assets will not be realized. The components of the total valuation allowance as of December 31, 2023 and 2022 were as follows (in thousands):


	2023		2022
Allowance related to:
Foreign tax credits	$	4,422	$	7,666
Federal net operating losses	3,783		3,783
State net operating loss carryforwards	4,809		9,928
Foreign net operating loss carryforwards	17,464		15,488
Deferred tax assets of a Canadian subsidiary	4,489		5,339
Realized and unrealized capital losses	305		305
Total valuation allowance	$	35,272	$	42,509

The Company files U.S. federal income tax returns as well as income tax returns in various states and foreign jurisdictions. The Company may be subject to examination by the Internal Revenue Service for calendar years 2018 through 2022. The Company may be subject to examination by Canadian federal and provincial authorities for calendar years 2016 through 2022 and by state and local revenue authorities for calendar years 2017 through 2022. The Company has ongoing U.S., state and local jurisdictional audits, as well as Canadian federal and provincial audits, all of which the Company believes will not result in material liabilities. The Company has not identified any material uncertain tax positions in the periods presented.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480990/946-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h)(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//740/tableOfContent

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-14

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-21

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 270
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482526/740-270-50-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 17
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-17

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I.5.Q1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479360/740-10-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 11.C)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479360/740-10-S99-2

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482603/740-30-50-2

+ Details

REMEDIAL LIABILITIES - Estimates (Details) $ in Thousands	12 Months Ended
REMEDIAL LIABILITIES - Estimates (Details) $ in Thousands	Dec. 31, 2023 USD ($) facility	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Site Contingency [Line Items]
Accrual for environmental loss contingencies	$ 111,243	$ 116,290	$ 111,873
% of Total	100.00%
Reasonably possible additional liabilities	$ 21,815
Accrual for environmental loss contingencies	5.00%
Loss contingency accrual	$ 32,400	37,100
Total remedial liabilities	111,243	116,290	$ 111,873
Legal and Administrative Proceedings
Site Contingency [Line Items]
Loss contingency accrual	25,000	$ 24,100
Inactive facilities not used in active conduct of the Company's business, most of which were assumed through prior period acquisitions (24 facilities)
Site Contingency [Line Items]
Accrual for environmental loss contingencies	$ 60,277
% of Total	54.20%
Reasonably possible additional liabilities	$ 11,585
Number of facility by type \| facility	24
Total remedial liabilities	$ 60,277
Facilities now used in active conduct of the Company's business (45 facilities)
Site Contingency [Line Items]
Accrual for environmental loss contingencies	$ 43,299
% of Total	38.90%
Reasonably possible additional liabilities	$ 9,080
Number of facility by type \| facility	45
Total remedial liabilities	$ 43,299
Superfund sites
Site Contingency [Line Items]
Accrual for environmental loss contingencies	$ 7,667
% of Total	6.90%
Reasonably possible additional liabilities	$ 1,150
Number of facility by type \| facility	13
Total remedial liabilities	$ 7,667
Superfund sites \| Various
Site Contingency [Line Items]
Accrual for environmental loss contingencies	$ 7,667
% of Total	6.90%
Reasonably possible additional liabilities	$ 1,150
Number of facility by type \| facility	13
Total remedial liabilities	$ 7,667
Closed incinerator and landfill \| Baton Rouge, LA
Site Contingency [Line Items]
Accrual for environmental loss contingencies	$ 27,883
% of Total	25.10%
Reasonably possible additional liabilities	$ 4,868
Total remedial liabilities	27,883
Closed incinerator \| Bridgeport, NJ
Site Contingency [Line Items]
Accrual for environmental loss contingencies	$ 17,472
% of Total	15.70%
Reasonably possible additional liabilities	$ 3,288
Total remedial liabilities	17,472
Idled incinerator \| Mercier, Quebec
Site Contingency [Line Items]
Accrual for environmental loss contingencies	$ 11,501
% of Total	10.30%
Reasonably possible additional liabilities	$ 1,735
Total remedial liabilities	11,501
Operating solvent recycling center \| Linden, NJ
Site Contingency [Line Items]
Accrual for environmental loss contingencies	$ 9,614
% of Total	8.60%
Reasonably possible additional liabilities	$ 2,187
Total remedial liabilities	9,614
All other incinerators, landfills, wastewater treatment facilities and service centers (65 facilities) \| Various
Site Contingency [Line Items]
Accrual for environmental loss contingencies	$ 37,106
% of Total	33.40%
Reasonably possible additional liabilities	$ 8,587
Number of facility by type \| facility	65
Total remedial liabilities	$ 37,106

SEGMENT REPORTING - Assets by Geographical Area (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Property, plant and equipment, net	$ 2,193,318	$ 1,980,302
Percent of property, plant and equipment, net	100.00%	100.00%
Permits and other intangibles, net	$ 602,797	$ 620,782
Percent of permits and other intangibles, net	100.00%	100.00%
Assets	$ 6,382,869	$ 6,129,707
United States
Segment Reporting Information [Line Items]
Property, plant and equipment, net	$ 1,972,221	$ 1,765,291
Percent of property, plant and equipment, net	89.90%	89.10%
Permits and other intangibles, net	$ 570,049	$ 585,887
Percent of permits and other intangibles, net	94.60%	94.40%
Assets	$ 5,786,937	$ 5,595,255
Canada and other foreign
Segment Reporting Information [Line Items]
Property, plant and equipment, net	$ 221,097	$ 215,011
Percent of property, plant and equipment, net	10.10%	10.90%
Assets	$ 595,932	$ 534,452
Canada
Segment Reporting Information [Line Items]
Permits and other intangibles, net	$ 32,748	$ 34,895
Percent of permits and other intangibles, net	5.40%	5.60%

- Definition

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 810
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (bb)
-Publisher FASB
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 810
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-Section 45
-Paragraph 25
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481231/810-10-45-25

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(g)(1)(ii))
-Publisher FASB
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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Reference 6: http://www.xbrl.org/2003/role/exampleRef
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Reference 7: http://www.xbrl.org/2003/role/exampleRef
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

SEGMENT REPORTING - Reconciliation of Third Party Revenues to Direct Revenues (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 5,409,152	$ 5,166,605	$ 3,805,566
Environmental Services
Segment Reporting Information [Line Items]
Total revenues	4,511,442	4,171,706	3,032,454
Safety-Kleen Sustainability Solutions
Segment Reporting Information [Line Items]
Total revenues	897,263	994,392	772,813
Operating segments \| Environmental Services
Segment Reporting Information [Line Items]
Total revenues	4,469,909	4,144,973	3,025,907
Operating segments \| Safety-Kleen Sustainability Solutions
Segment Reporting Information [Line Items]
Total revenues	938,796	1,021,125	779,360
Intersegment revenues (expense), net \| Environmental Services
Segment Reporting Information [Line Items]
Total revenues	41,533	26,733	6,547
Intersegment revenues (expense), net \| Safety-Kleen Sustainability Solutions
Segment Reporting Information [Line Items]
Total revenues	(41,533)	(26,733)	(6,547)
Corporate Items
Segment Reporting Information [Line Items]
Total revenues	$ 447	$ 507	$ 299

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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+ Details

SIGNIFICANT ACCOUNTING POLICIES - Goodwill (Details)	12 Months Ended
SIGNIFICANT ACCOUNTING POLICIES - Goodwill (Details)	Dec. 31, 2023 reporting_unit
Accounting Policies [Abstract]
Number of reporting units	4

GOODWILL AND OTHER INTANGIBLE ASSETS (Tables)

12 Months Ended

Dec. 31, 2023

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of Changes to Goodwill

The changes in goodwill for the years ended December 31, 2023 and 2022 were as follows (in thousands):


	Environmental Services		Safety-Kleen Sustainability Solutions		Total
Balance at January 1, 2022	$	1,085,534	$	141,508	$	1,227,042
Increase from current period acquisitions	—		34,510		34,510
Measurement period adjustments from prior period acquisition	(6,762)		—		(6,762)
Decrease from disposition of business	(4,412)		—		(4,412)

Foreign currency translation	(2,514)		(986)		(3,500)
Balance at December 31, 2022	$	1,071,846	$	175,032	$	1,246,878
Increase from current period acquisition	39,346		—		39,346
Measurement period adjustments from prior period acquisitions	—		360		360

Foreign currency translation	821		331		1,152
Balance at December 31, 2023	$	1,112,013	$	175,723	$	1,287,736

Schedule of Finite-Lived Intangible Assets by Major Class

As of December 31, 2023 and 2022, the Company's intangible assets consisted of the following (in thousands):


	December 31, 2023						December 31, 2022
	Cost		Accumulated Amortization		Net		Cost		Accumulated Amortization		Net
Permits	$	191,747	$	117,556	$	74,191	$	188,373	$	109,036	$	79,337
Customer and supplier relationships	604,994		258,879		346,115		583,709		229,368		354,341
Other intangible assets	100,068		37,862		62,206		89,388		24,818		64,570
Total amortizable permits and other intangible assets	896,809		414,297		482,512		861,470		363,222		498,248
Trademarks and trade names	120,285		—		120,285		122,534		—		122,534
Total permits and other intangible assets	$	1,017,094	$	414,297	$	602,797	$	984,004	$	363,222	$	620,782

Schedule of Indefinite-Lived Intangible Assets

As of December 31, 2023 and 2022, the Company's intangible assets consisted of the following (in thousands):


	December 31, 2023						December 31, 2022
	Cost		Accumulated Amortization		Net		Cost		Accumulated Amortization		Net
Permits	$	191,747	$	117,556	$	74,191	$	188,373	$	109,036	$	79,337
Customer and supplier relationships	604,994		258,879		346,115		583,709		229,368		354,341
Other intangible assets	100,068		37,862		62,206		89,388		24,818		64,570
Total amortizable permits and other intangible assets	896,809		414,297		482,512		861,470		363,222		498,248
Trademarks and trade names	120,285		—		120,285		122,534		—		122,534
Total permits and other intangible assets	$	1,017,094	$	414,297	$	602,797	$	984,004	$	363,222	$	620,782

Schedule of Expected Amortization for the Net Carrying Amount of Finite Lived Intangible Assets

The expected amortization of the net carrying amount of finite-lived intangible assets at December 31, 2023 was as follows (in thousands):


Years ending December 31,	Expected Amortization
2024	$	46,639
2025	43,231
2026	41,385
2027	39,245
2028	37,975
Thereafter	274,037
	$	482,512

- Definition

Tabular disclosure of goodwill by reportable segment and in total which includes a rollforward schedule.

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+ Details

REVENUES - Contract Balances (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Revenue from Contract with Customer [Abstract]
Receivables	$ 983,111	$ 964,603
Contract assets (unbilled receivables)	107,859	107,010
Contract liabilities (deferred revenue)	$ 95,230	$ 94,094

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenues:
Total revenues	$ 5,409,152	$ 5,166,605	$ 3,805,566
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues	3,746,124	3,543,930	2,609,837
Selling, general and administrative expenses	671,161	627,391	537,962
Accretion of environmental liabilities	13,667	12,943	11,745
Depreciation and amortization	365,761	347,594	298,135
Income from operations	612,439	634,747	347,887
Other income (expense), net	2,315	2,472	(515)
Loss on early extinguishment of debt	(2,880)	(422)	0
Gain on sale of business	0	8,864	0
Interest expense, net of interest income of $11,930, $4,607 and $2,218, respectively	(108,595)	(107,663)	(77,657)
Income from operations before provision for income taxes	503,279	537,998	269,715
Provision for income taxes	125,423	126,254	66,468
Net income	$ 377,856	$ 411,744	$ 203,247
Earnings per share:
Basic (in dollars per share)	$ 6.99	$ 7.59	$ 3.73
Diluted (in dollars per share)	$ 6.95	$ 7.56	$ 3.71
Shares used to compute earnings per share — Basic (in shares)	54,071	54,223	54,514
Shares used to compute earnings per share — Diluted (in shares)	54,382	54,487	54,761
Service revenues
Revenues:
Total revenues	$ 4,449,542	$ 4,133,184	$ 3,048,019
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues	3,054,594	2,892,726	2,105,043
Product revenues
Revenues:
Total revenues	959,610	1,033,421	757,547
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues	$ 691,530	$ 651,204	$ 504,794

EARNINGS PER SHARE - Antidilutive Securities Excluded from Computation of Earnings Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of performance stock awards that performance criteria not attained (in shares)	88,876	0	14,237
Restricted Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	12,247	10,250	67,981

COMMITMENTS AND CONTINGENCIES - Federal, State and Provincial Enforcement Actions (Details)	Dec. 31, 2023 proceeding
Federal, State, and Provincial Enforcement Actions
Federal and State Enforcement Actions
Number of proceedings	0

Insider Trading Arrangements	3 Months Ended
Insider Trading Arrangements	Dec. 31, 2023
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

STOCK-BASED COMPENSATION - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 20.7	$ 26.8	$ 18.8
Total income tax benefit from stock-based compensation	$ 3.5	$ 5.0	$ 3.5
Minimum \| Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)	3 years
Maximum \| Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)	5 years
2020 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)				2,500,000

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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+ Details

ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES (Tables)

12 Months Ended

Dec. 31, 2023

Payables and Accruals [Abstract]

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	December 31, 2023		December 31, 2022
Accrued insurance	$	107,658	$	92,909
Accrued interest	33,857		20,033
Accrued compensation and benefits	113,236		123,226
Accrued income, real estate, sales and other taxes	44,752		61,442

Accrued other	97,654		99,106
	$	397,157	$	396,716

INCOME TAXES - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Provision for doubtful accounts	$ 10,882	$ 11,544
Closure, post-closure and remedial liabilities	31,944	31,837
Operating lease liabilities	46,784	42,255
Accrued expenses	14,963	19,311
Accrued compensation and benefits	15,058	20,171
Net operating loss carryforwards	39,042	41,585
Tax credit carryforwards	6,531	8,903
Stock-based compensation	3,516	3,988
Other	4,843	7,487
Total deferred tax assets	173,563	187,081
Deferred tax liabilities:
Property, plant and equipment	(284,997)	(281,131)
Operating lease right-of-use assets	(46,584)	(41,939)
Interest rate swap asset	(9,576)	(17,587)
Permits and other intangible assets	(130,391)	(132,681)
Prepaid expenses	(12,372)	(12,088)
Total deferred tax liabilities	(483,920)	(485,426)
Total net deferred tax liability before valuation allowance	(310,357)	(298,345)
Less valuation allowance	(35,272)	(42,509)
Net deferred tax liabilities	(345,629)	$ (340,854)
State net operating loss carryforwards, begin to expire in 2023	229,900
Federal operating loss carryforwards, begin to expire in 2024	26,600
Foreign net operating loss carryforwards, begin to expire in 2023	83,200
Foreign tax credit carryforwards, begin to expire between 2023 and 2024	$ 4,400

GOODWILL AND OTHER INTANGIBLE ASSETS - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization expense of permits and other intangible assets	$ 50.3	$ 50.2	$ 34.7

SEGMENT REPORTING - Schedule of Segment Reporting Information, by Segment (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Property, plant and equipment, net	$ 2,193,318	$ 1,980,302
Goodwill	1,287,736	1,246,878	$ 1,227,042
Permits and other intangibles, net	602,797	620,782
Permits and other intangibles, net	1,890,533	1,867,660
Environmental Services
Segment Reporting Information [Line Items]
Goodwill	1,112,013	1,071,846	1,085,534
Safety-Kleen Sustainability Solutions
Segment Reporting Information [Line Items]
Goodwill	175,723	175,032	$ 141,508
Operating segments \| Environmental Services
Segment Reporting Information [Line Items]
Property, plant and equipment, net	1,651,026	1,467,641
Goodwill	1,112,013	1,071,846
Permits and other intangibles, net	454,919	462,050
Permits and other intangibles, net	1,566,932	1,533,896
Operating segments \| Safety-Kleen Sustainability Solutions
Segment Reporting Information [Line Items]
Property, plant and equipment, net	411,415	398,660
Goodwill	175,723	175,032
Permits and other intangibles, net	147,878	158,732
Permits and other intangibles, net	323,601	333,764
Corporate, non-segment
Segment Reporting Information [Line Items]
Property, plant and equipment, net	$ 130,877	$ 114,001

- Definition

+ References + Details

COMMITMENTS AND CONTINGENCIES (Tables)

12 Months Ended

Dec. 31, 2023

Commitments and Contingencies Disclosure [Abstract]

Schedule of Self-Insurance Liabilities Anticipated Payments


Years ending December 31,
2024	$	33,954
2025	19,675
2026	14,663
2027	9,153
2028	6,275
Thereafter	9,650
Undiscounted self-insurance liabilities	93,370
Less: Discount	(10,670)
Total self-insurance liabilities (included in accrued expenses and other current liabilities)	$	82,700

LEASES - Supplemental Finance Lease Balance Sheet information (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
ROU assets:
ROU assets (Property, plant and equipment, net)	$ 126,292	$ 96,207
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Property, plant and equipment, net	Property, plant and equipment, net
Current portion of lease liabilities:
Current portion of lease liabilities (Accrued expenses and other current liabilities)	$ 16,975	$ 12,767
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	Accrued expenses and other current liabilities	Accrued expenses and other current liabilities
Long-term portion of lease liabilities:
Long-term portion of lease liabilities (Other long-term liabilities)	$ 113,486	$ 86,390
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other long-term liabilities	Other long-term liabilities

INVENTORIES AND SUPPLIES (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Inventory [Line Items]
Inventories and supplies	$ 327,511	$ 324,994
Oil and oil related products
Inventory [Line Items]
Inventories and supplies	118,600	151,519
Supplies
Inventory [Line Items]
Inventories and supplies	177,217	143,743
Solvent and solutions
Inventory [Line Items]
Inventories and supplies	11,795	11,994
Other
Inventory [Line Items]
Inventories and supplies	$ 19,899	$ 17,738

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities:
Net income	$ 377,856	$ 411,744	$ 203,247
Adjustments to reconcile net income to net cash from operating activities:
Depreciation and amortization	365,761	347,594	298,135
Allowance for doubtful accounts	5,956	7,783	8,018
Amortization of deferred financing costs and debt discount	5,309	6,301	4,245
Accretion of environmental liabilities	13,667	12,943	11,745
Changes in environmental liability estimates	4,828	8,272	2,979
Deferred income taxes	12,685	17,549	1,482
Other (income) expense, net	(2,315)	(2,472)	515
Stock-based compensation	20,703	26,844	18,839
Loss on early extinguishment of debt	2,880	422	0
Gain on sale of business	0	(8,864)	0
Environmental expenditures	(28,960)	(13,946)	(15,506)
Changes in assets and liabilities, net of acquisitions:
Accounts receivable and unbilled accounts receivable	2,453	(201,087)	(96,551)
Inventories and supplies	(4,312)	(74,547)	(31,689)
Other current and non-current assets	(22,645)	(17,303)	9,268
Accounts payable	(27,425)	74,460	108,398
Other current and long-term liabilities	8,111	30,521	22,872
Net cash from operating activities	734,552	626,214	545,997
Cash flows used in investing activities:
Additions to property, plant and equipment	(422,300)	(345,056)	(241,856)
Proceeds from sale and disposal of fixed assets	9,650	8,779	22,156
Acquisitions, net of cash acquired	(119,596)	(86,278)	(1,253,232)
Proceeds from sale of business, net of transaction costs	750	16,811	0
Additions to intangible assets including costs to obtain or renew permits	(2,649)	(1,966)	(3,848)
Purchases of available-for-sale securities	(158,264)	(49,845)	(129,234)
Proceeds from sale of available-for-sale securities	117,359	68,611	98,412
Net cash used in investing activities	(575,050)	(388,944)	(1,507,602)
Cash flows (used in) from financing activities:
Change in uncashed checks	2,759	552	(1,806)
Tax payments related to withholdings on vested restricted stock	(13,838)	(8,801)	(10,805)
Repurchases of common stock	(51,164)	(50,183)	(54,410)
Deferred financing costs paid	(6,736)	(410)	(13,737)
Payments on finance leases	(15,937)	(12,821)	(8,458)
Principal payments on debt	(623,975)	(115,652)	(7,535)
Proceeds from issuance of debt, net of discount	500,000	0	995,000
Borrowing from revolving credit facility	114,000	0	0
Payment on revolving credit facility	(114,000)	0	0
Net cash (used in) from financing activities	(208,891)	(187,315)	898,249
Effect of exchange rate change on cash	1,484	(9,927)	(3,170)
(Decrease) increase in cash and cash equivalents	(47,905)	40,028	(66,526)
Cash and cash equivalents, beginning of year	492,603	452,575	519,101
Cash and cash equivalents, end of year	444,698	492,603	452,575
Cash payments for interest and income taxes:
Interest paid	114,560	105,643	73,440
Income taxes paid, net of refunds	132,314	78,526	65,192
Non-cash investing activities:
Property, plant and equipment accrued	52,376	30,950	19,264
Remedial liability assumed in acquisition of property, plant and equipment	$ 0	$ 8,092	$ 0

SIGNIFICANT ACCOUNTING POLICIES - Property, Plant and Equipment, net (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Line Items]
Impairment charges related to long-lived assets	$ 0	$ 0	$ 0
Minimum \| Buildings
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	20 years
Minimum \| Leasehold and building improvements
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	2 years
Minimum \| Vehicles
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	2 years
Minimum \| Capitalized software and computer equipment
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	3 years
Minimum \| Containers and railcars
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	8 years
Minimum \| All other equipment
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	4 years
Minimum \| Furniture and fixtures
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	5 years
Maximum \| Buildings
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	42 years
Maximum \| Leasehold and building improvements
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	45 years
Maximum \| Vehicles
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	15 years
Maximum \| Capitalized software and computer equipment
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	5 years
Maximum \| Containers and railcars
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	16 years
Maximum \| All other equipment
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	30 years
Maximum \| Furniture and fixtures
Property, Plant and Equipment [Line Items]
Weighted average remaining useful life (in years)	8 years

GOODWILL AND OTHER INTANGIBLE ASSETS

12 Months Ended

Dec. 31, 2023

Goodwill and Intangible Assets Disclosure [Abstract]

GOODWILL AND OTHER INTANGIBLE ASSETS

The changes in goodwill for the years ended December 31, 2023 and 2022 were as follows (in thousands):


	Environmental Services		Safety-Kleen Sustainability Solutions		Total
Balance at January 1, 2022	$	1,085,534	$	141,508	$	1,227,042
Increase from current period acquisitions	—		34,510		34,510
Measurement period adjustments from prior period acquisition	(6,762)		—		(6,762)
Decrease from disposition of business	(4,412)		—		(4,412)

Foreign currency translation	(2,514)		(986)		(3,500)
Balance at December 31, 2022	$	1,071,846	$	175,032	$	1,246,878
Increase from current period acquisition	39,346		—		39,346
Measurement period adjustments from prior period acquisitions	—		360		360

Foreign currency translation	821		331		1,152
Balance at December 31, 2023	$	1,112,013	$	175,723	$	1,287,736

The Company assesses goodwill for impairment on an annual basis as of December 31 or at an interim date when it is more likely than not that events or changes in the business environment (“triggering events”) would reduce the fair value of a reporting unit below its carrying value. The Company did not identify any triggering events in the years presented.

During the fourth quarter of 2023, the Company changed reporting units, separating the historical Environmental Sales and Service reporting unit into two reporting units, Environmental Sales and Service and Industrial Services. This change required goodwill in the Environmental Sales and Services reporting unit to be allocated to the new reporting units based on relative fair value. The Company tested goodwill of the new reporting units for impairment both before and following the change in reporting unit structure, and no impairment was identified.

The Company conducted its annual impairment test of goodwill as of December 31, 2023 and determined that no adjustment to the carrying value of goodwill for any reporting unit was then necessary because the fair values of the reporting units exceeded their respective carrying values. The fair value of all reporting units was determined using an income approach based upon estimates of future discounted cash flows. The resulting estimates of fair value were validated through the consideration of other factors such as the fair value of comparable companies to the reporting units and a reconciliation of the sum of all estimated fair values of the reporting units to the Company’s overall market capitalization. In all cases, the estimated fair values of the reporting units significantly exceeded the respective carrying values.

Significant judgments and unobservable inputs, categorized as Level 3 in the fair value hierarchy, are inherent in the impairment tests performed and include assumptions about the amount and timing of expected future cash flows, growth rates, and the determination of appropriate discount rates. Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date. The Company believes that the assumptions used in its impairment tests are reasonable, but variations in any of the assumptions may result in different measurements of fair values.

The impacts of any adverse business and market conditions which may impact the overall performance of the Company's reporting units will continue to be monitored. If the Company's reporting units do not achieve the financial performance that the Company expects, or if there is a significant prolonged change in demand for the Company's products and services, it is possible that goodwill impairment charges may result. Therefore, there can be no assurance that future events will not result in an impairment of goodwill.

As of December 31, 2023 and 2022, the Company's intangible assets consisted of the following (in thousands):


	December 31, 2023						December 31, 2022
	Cost		Accumulated Amortization		Net		Cost		Accumulated Amortization		Net
Permits	$	191,747	$	117,556	$	74,191	$	188,373	$	109,036	$	79,337
Customer and supplier relationships	604,994		258,879		346,115		583,709		229,368		354,341
Other intangible assets	100,068		37,862		62,206		89,388		24,818		64,570
Total amortizable permits and other intangible assets	896,809		414,297		482,512		861,470		363,222		498,248
Trademarks and trade names	120,285		—		120,285		122,534		—		122,534
Total permits and other intangible assets	$	1,017,094	$	414,297	$	602,797	$	984,004	$	363,222	$	620,782

During the periods presented, there were no events or changes in circumstances which would indicate that the carrying values of the Company's asset groups would not be recoverable and thus no impairment charges were recorded. If expectations of future cash flows were to decrease in the future as a result of worse than expected or prolonged periods of depressed activity, future impairments may become evident.

Amortization expense of permits, customer and supplier relationships and other intangible assets for the years ended December 31, 2023, 2022 and 2021 were $50.3 million, $50.2 million and $34.7 million, respectively.

The expected amortization of the net carrying amount of finite-lived intangible assets at December 31, 2023 was as follows (in thousands):


Years ending December 31,	Expected Amortization
2024	$	46,639
2025	43,231
2026	41,385
2027	39,245
2028	37,975
Thereafter	274,037
	$	482,512

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Account receivable, allowances aggregating	$ 42,209	$ 45,253
Closure and post-closure liabilities, current portion	13,556	13,205
Remedial liabilities, current portion	$ 13,358	$ 9,918
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, authorized shares (in shares)	80,000,000	80,000,000
Common stock, issued shares (in shares)	53,929,703	54,064,797
Common stock, outstanding shares (in shares)	53,929,703	54,064,797

REVENUES (Tables)

12 Months Ended

Dec. 31, 2023

Revenue from Contract with Customer [Abstract]

Schedule of Disaggregation of Revenue

The following tables present the Company's third-party revenue disaggregated by source of revenue and geography (in thousands):


	For the year ended December 31, 2023
	Environmental Services		Safety-Kleen Sustainability Solutions		Corporate		Total
Primary Geographical Markets
United States	$	4,022,394	$	846,339	$	447	$	4,869,180
Canada	447,515		92,457		—		539,972
Total third-party revenues	$	4,469,909	$	938,796	$	447	$	5,409,152

Sources of Revenue
Technical Services	$	1,563,847	$	—	$	—	$	1,563,847
Industrial Services and Other	1,418,938		—		447		1,419,385
Field and Emergency Response Services	609,913		—		—		609,913
Safety-Kleen Environmental Services	877,211		224,830		—		1,102,041
Safety-Kleen Oil	—		713,966		—		713,966
Total third-party revenues	$	4,469,909	$	938,796	$	447	$	5,409,152


	For the year ended December 31, 2022
	Environmental Services		Safety-Kleen Sustainability Solutions		Corporate		Total
Primary Geographical Markets
United States	$	3,675,880	$	899,780	$	507	$	4,576,167
Canada	469,093		121,345		—		590,438
Total third-party revenues	$	4,144,973	$	1,021,125	$	507	$	5,166,605

Sources of Revenue
Technical Services	$	1,495,264	$	—	$	—	$	1,495,264
Industrial Services and Other	1,311,072		—		507		1,311,579
Field and Emergency Response Services	575,300		—		—		575,300
Safety-Kleen Environmental Services	763,337		199,360		—		962,697
Safety-Kleen Oil	—		821,765		—		821,765
Total third-party revenues	$	4,144,973	$	1,021,125	$	507	$	5,166,605


	For the year ended December 31, 2021
	Environmental Services		Safety-Kleen Sustainability Solutions		Corporate		Total
Primary Geographical Markets
United States	$	2,631,112	$	693,542	$	299	$	3,324,953
Canada	394,795		85,818		—		480,613
Total third-party revenues	$	3,025,907	$	779,360	$	299	$	3,805,566

Sources of Revenue
Technical Services	$	1,209,624	$	—	$	—	$	1,209,624
Industrial Services and Other	705,999		—		299		706,298
Field and Emergency Response Services	466,380		—		—		466,380
Safety-Kleen Environmental Services	643,904		161,587		—		805,491
Safety-Kleen Oil	—		617,773		—		617,773
Total third-party revenues	$	3,025,907	$	779,360	$	299	$	3,805,566

Schedule of Contract Balances

Contract Balances


(in thousands)	December 31, 2023		December 31, 2022
Receivables	$	983,111	$	964,603
Contract assets (unbilled receivables)	107,859		107,010
Contract liabilities (deferred revenue)	95,230		94,094

BUSINESS COMBINATIONS

12 Months Ended

Dec. 31, 2023

Business Combination and Asset Acquisition [Abstract]

BUSINESS COMBINATIONS

Proposed 2024 Acquisition

On February 2, 2024, the Company signed a definitive agreement with Gryphon Investors, Inc. to acquire HEPACO, a leading provider of specialized environmental and emergency response services in the Eastern United States, for $400.0 million in cash, subject to working capital and other adjustments. The Company intends to fund the acquisition with a combination of available cash and incremental borrowings under our term loan facility. The acquisition will expand field services operations within the Environmental Services segment and is expected to close in the first half of 2024, subject to regulatory approval and other customary closing conditions.

2023 Acquisition

On March 31, 2023, the Company acquired Thompson Industrial Services, LLC ("Thompson Industrial") for an all-cash purchase price of $110.9 million, net of cash acquired. The operations of Thompson Industrial expand the Environmental Services segment's industrial service operations in the southeastern region of the United States.

The preliminary allocation of the purchase price is provisional and was based on estimates of the fair value of assets acquired and liabilities assumed as of the acquisition date. The Company continues to obtain information to complete the valuation of these balances and the associated income tax accounting. Measurement period adjustments will reflect new information obtained about facts and circumstances that existed as of the acquisition date. The following table summarizes the preliminary determination and recognition of assets acquired and liabilities assumed (in thousands):


	At Acquisition Date As Reported December 31, 2023
Accounts receivable	$	25,233
Inventories and supplies	228
Prepaid expenses and other current assets	1,302
Property, plant and equipment	26,719
Permits and other intangibles	28,900
Operating lease right-of-use assets	4,716
Other long-term assets	72
Current liabilities	(10,385)
Current portion of operating lease liabilities	(1,653)
Operating lease liabilities, less current portion	(3,063)

Other long-term liabilities	(560)
Total identifiable net assets	71,509
Goodwill	39,346
Total purchase price	$	110,855

Permits and other intangible assets acquired include customer relationships, trademarks/tradenames and non-compete agreements and are anticipated to have estimated useful lives of between five and 15 years with a weighted average useful life of approximately 13 years. The excess of the total purchase price, which includes the aggregate cash consideration paid in excess of the fair value of the tangible and intangible assets acquired and liabilities assumed, was recorded as goodwill. The goodwill recognized is attributable to the expected operating synergies, assembled workforce and growth potential that the Company expects to realize from the acquisition. Goodwill generated from the acquisition is deductible for tax purposes.

The operations included in the Company's financial statements for the year ended December 31, 2023, and pro forma revenue and earnings amounts on a combined basis as if this acquisition had been completed on January 1, 2022 are immaterial to the consolidated financial statements of the Company.

2022 Acquisitions

On June 17, 2022, the Company acquired a privately-owned company for an all-cash purchase price of approximately $78.9 million, net of cash acquired. The operations of the newly acquired company expand the SKSS segment's waste oil collection capabilities and re-refining business throughout the southeastern region of the United States, including the addition of a re-refinery in Georgia.

The Company finalized the purchase accounting for this acquisition in the second quarter of 2023. The allocation of the purchase price was based on estimates of the fair value and assets acquired and liabilities assumed as of June 17, 2022. The following table summarizes the final determination and recognition of assets acquired and liabilities assumed (in thousands):


	Preliminary Allocation As Reported December 31, 2022		Measurement Period Adjustments		Final Allocation As Reported December 31, 2023
Accounts receivable	$	1,111	$	(22)	$	1,089
Inventories and supplies	5,816		(71)		5,745
Prepaid expenses and other current assets	144		—		144
Property, plant and equipment	19,605		2,626		22,231
Permits and other intangibles	23,500		—		23,500
Operating lease right-of-use assets	585		—		585
Other long-term assets	13		—		13
Current liabilities	(3,271)		(104)		(3,375)
Current portion of operating lease liabilities	(186)		—		(186)
Operating lease liabilities, less current portion	(399)		—		(399)

Other long-term liabilities	(55)		(2,626)		(2,681)
Total identifiable net assets	46,863		(197)		46,666
Goodwill	32,015		197		32,212
Total purchase price	$	78,878	$	—	$	78,878

Permits and other intangible assets acquired include supplier relationships, permits, customer relationships and trademarks/tradenames and are anticipated to have estimated useful lives of between five and 20 years with a weighted average useful life of approximately 18 years. The excess of the total purchase price, which includes the aggregate cash consideration paid in excess of the fair value of the tangible and intangible assets acquired, was recorded as goodwill. The goodwill recognized is attributable to the expected operating synergies, assembled workforce and growth potential that the Company expects to realize from the acquisition. Goodwill generated from the acquisition is deductible for tax purposes.

The operations included in the Company's financial statements for the period ended December 31, 2022 and pro forma revenue and earnings amounts on a combined basis as if this acquisition had been completed on January 1, 2021 were not material in 2022 to the consolidated financial statements of the Company.

On December 9, 2022, the Company acquired a privately-owned business for $12.6 million cash consideration. The acquired company expands the SKSS segment's oil collection operations in the southeastern United States. In connection with this acquisition, goodwill of $2.7 million was recognized. The results of operations for the acquired business were not material in 2022 to the consolidated financial statements of the Company.

HydroChemPSC

On October 8, 2021, the Company completed its acquisition of LJ Energy Services Intermediate Holding Corp. and its subsidiaries (collectively, “HydroChemPSC”), a privately owned company, for an all-cash purchase price of approximately $1.23 billion. HydroChemPSC is a leading U.S. provider of industrial cleaning, specialty maintenance and utilities services. These operations enhance and have been fully integrated into the Company's Environmental Services segment. In the first quarter of 2022, the Company received $5.0 million after finalizing the acquisition date working capital balances, which decreased the overall purchase price.

The Company finalized the purchase accounting for the acquisition of HydroChemPSC in the third quarter of 2022. The allocation of the purchase price was based on estimates of the fair value of assets acquired and liabilities assumed as of October

8, 2021. The following table summarizes the final determination and recognition of assets acquired and liabilities assumed (in thousands):


	Final Allocation
Accounts receivable, including unbilled receivables	$	131,516
Inventories and supplies	3,162
Prepaid expenses and other current assets	16,291
Property, plant and equipment	314,397
Other intangibles	289,000
Operating lease right-of-use assets	34,415
Other long-term assets	962
Current liabilities	(118,854)
Current portion of operating lease liabilities	(11,277)
Operating lease liabilities, less current portion	(26,344)
Deferred tax liabilities	(80,386)
Other long-term liabilities	(4,170)
Total identifiable net assets	548,712
Goodwill (i)	676,701
Total purchase price	$	1,225,413

_____________

(i) Goodwill represents the excess of the fair value of the net assets acquired over the purchase price. Goodwill of $676.7 million is attributable to the future economic benefits arising from the acquired operations, synergies and the acquired workforce of HydroChemPSC. None of the goodwill related to this acquisition will be deductible for tax purposes. Final Goodwill was assigned to the Environmental Sales and Service reporting unit, see Note 8, "Goodwill and Other Intangible Assets" for details on reporting unit change as of December 31, 2023.

Unaudited Pro Forma Financial Information

The following table presents unaudited pro forma combined summary financial information for the year ended December 31, 2021, and assumes the acquisition of HydroChemPSC occurred on January 1, 2020 (in thousands):


	2021
Pro forma combined revenues	$	4,380,724
Pro forma combined net income	229,807

The pro forma results do not include any costs incurred directly attributable to the acquisition of HydroChemPSC. The pro forma results do reflect impacts resulting from the issuance of $1.0 billion secured senior term loans issued in connection with the acquisition assuming interest rates in effect at the time of the acquisition.

This pro forma financial information is not necessarily indicative of the Company's consolidated operating results that would have been reported had the transactions been completed as described herein, nor is such information necessarily indicative of the Company's consolidated results for any future period. Interest expense used in calculating the pro forma net income in both periods did not contemplate the interest rate swap that the Company put in place in early 2022. See Note 12, "Financing Arrangements."

Other 2021 Acquisition Activity

On March 27, 2021, the Company acquired a privately-owned business for $22.8 million cash consideration. The acquired company increases the SKSS segment's network within the south central United States. In connection with this acquisition, a goodwill amount of $16.3 million was recognized. The results of operations for this acquired business were not material in 2021.

On June 29, 2021, the Company signed a definitive agreement with Vertex Energy, Inc. ("Vertex") to acquire certain assets related to Vertex's used motor oil collection and re-refinery business in an all-cash transaction for $140.0 million, subject

to working capital and other adjustments. On January 25, 2022, Vertex and the Company mutually agreed to terminate the planned acquisition. Pursuant to the agreement, Vertex paid Clean Harbors a breakup fee of $3.0 million in early 2022 in connection with the termination.

PROPERTY, PLANT, AND EQUIPMENT

12 Months Ended

Dec. 31, 2023

Property, Plant and Equipment [Abstract]

PROPERTY, PLANT, AND EQUIPMENT

PROPERTY, PLANT AND EQUIPMENT

Property, plant and equipment consisted of the following (in thousands):


	December 31, 2023		December 31, 2022
Land	$	174,891	$	172,579
Asset retirement costs (non-landfill)	27,167		22,001
Landfill assets	253,180		232,872
Buildings and improvements (1)	630,525		591,397

Vehicles (2)	1,276,567		1,112,188
Equipment (3)	2,388,370		2,195,064

Construction in progress	213,601		140,328
	4,964,301		4,466,429
Less - accumulated depreciation and amortization	2,770,983		2,486,127
Total property, plant and equipment, net	$	2,193,318	$	1,980,302

___________________________________

(1) Balances inclusive of gross ROU assets classified as finance leases of $8.0 million.

(2) Balances inclusive of gross ROU assets classified as finance leases of $151.7 million and $106.7 million, respectively.

(3) Balances inclusive of gross ROU assets classified as finance leases of $9.2 million.

Depreciation expense, inclusive of landfill and finance lease amortization was $315.5 million, $297.4 million and $263.4 million for the years ended December 31, 2023, 2022 and 2021, respectively. The Company recorded $6.6 million and $3.4

million capitalized interest in 2023 and 2022, respectively, mainly due to the construction of a new incinerator in Kimball, Nebraska. Capitalized interest in 2021 was negligible.

SEGMENT REPORTING

12 Months Ended

Dec. 31, 2023

Segment Reporting [Abstract]

SEGMENT REPORTING

Segment reporting is prepared on the same basis that the Company's chief operating decision maker, which is a committee comprised of the Company's Co-Chief Executive Officers, manages the business, makes operating decisions and assesses performance. The Company is managed and reports as two operating segments; (i) the Environmental Services segment and (ii) the Safety-Kleen Sustainability Solutions segment.

Third-party revenue is revenue billed to outside customers by a particular segment. Direct revenues is revenue allocated to the segment providing the product or service. Intersegment revenues represent the sharing of third-party revenues among the segments based on products and services provided by each segment as if the products and services were sold directly to the third-party. The intersegment revenues are shown net. The operations not managed through the Company’s operating segments described above are recorded as “Corporate Items.”

The following tables reconcile third-party revenues to direct revenues for the years ended December 31, 2023, 2022 and 2021 (in thousands):


	For the year ended December 31, 2023
	Environmental Services		Safety-Kleen Sustainability Solutions		Corporate Items		Total
Third-party revenues	$	4,469,909	$	938,796	$	447	$	5,409,152
Intersegment revenues (expense), net	41,533		(41,533)		—		—
Direct revenues	$	4,511,442	$	897,263	$	447	$	5,409,152


	For the year ended December 31, 2022
	Environmental Services		Safety-Kleen Sustainability Solutions		Corporate Items		Total
Third-party revenues	$	4,144,973	$	1,021,125	$	507	$	5,166,605
Intersegment revenues (expense), net	26,733		(26,733)		—		—
Direct revenues	$	4,171,706	$	994,392	$	507	$	5,166,605


	For the year ended December 31, 2021
	Environmental Services		Safety-Kleen Sustainability Solutions		Corporate Items		Total
Third-party revenues	$	3,025,907	$	779,360	$	299	$	3,805,566
Intersegment revenues (expense), net	6,547		(6,547)		—		—
Direct revenues	$	3,032,454	$	772,813	$	299	$	3,805,566

The primary financial measure by which the Company evaluates the performance of its segments is Adjusted EBITDA, which consists of net income plus accretion of environmental liabilities, stock-based compensation, depreciation and amortization, net interest expense, loss on early extinguishment of debt and provision for income taxes and excludes other gains, losses and non-cash charges not deemed representative of fundamental segment results and other (income) expense, net. Transactions between the segments are accounted for at the Company’s best estimate based on similar transactions with outside customers.

The following table presents Adjusted EBITDA information used by management by reported segment (in thousands):


	For the years ended December 31,
	2023		2022		2021
Adjusted EBITDA:
Environmental Services	$	1,101,608	$	953,053	$	659,718
Safety-Kleen Sustainability Solutions	172,873		306,327		227,354
Corporate Items	(261,911)		(237,252)		(210,466)
Total	1,012,570		1,022,128		676,606
Reconciliation to Consolidated Statements of Operations:

Accretion of environmental liabilities	13,667		12,943		11,745
Stock-based compensation	20,703		26,844		18,839
Depreciation and amortization	365,761		347,594		298,135

Income from operations	612,439		634,747		347,887
Other (income) expense, net	(2,315)		(2,472)		515
Loss on early extinguishment of debt	2,880		422		—
Gain on sale of business	—		(8,864)		—
Interest expense, net of interest income	108,595		107,663		77,657
Income from operations before provision for income taxes	$	503,279	$	537,998	$	269,715

The following table presents assets by reported segment and in the aggregate (in thousands):


	December 31, 2023		December 31, 2022
Property, plant and equipment, net
Environmental Services	$	1,651,026	$	1,467,641
Safety-Kleen Sustainability Solutions	411,415		398,660
Corporate Items	130,877		114,001
Total property, plant and equipment, net	$	2,193,318	$	1,980,302

Goodwill and Permits and other intangibles, net
Environmental Services
Goodwill	$	1,112,013	$	1,071,846
Permits and other intangibles, net	454,919		462,050
Total Environmental Services	1,566,932		1,533,896

Safety-Kleen Sustainability Solutions
Goodwill	$	175,723	$	175,032
Permits and other intangibles, net	147,878		158,732
Total Safety-Kleen Sustainability Solutions	323,601		333,764

Total	$	1,890,533	$	1,867,660

Geographic Information

As of December 31, 2023 and 2022, the Company had property, plant and equipment, net of depreciation and amortization and permits and other intangible assets, net of amortization in the following geographic locations (in thousands):


	December 31, 2023				December 31, 2022
	Total		% of Total		Total		% of Total
Property, plant and equipment, net
United States	$	1,972,221	89.9	%	$	1,765,291	89.1	%
Canada and other foreign	221,097		10.1		215,011		10.9
Total property, plant and equipment, net	$	2,193,318	100.0	%	$	1,980,302	100.0	%

Permits and other intangibles, net
United States	$	570,049	94.6	%	$	585,887	94.4	%
Canada	32,748		5.4		34,895		5.6
Total permits and other intangibles, net	$	602,797	100.0	%	$	620,782	100.0	%

The following table presents the total assets by geographical area (in thousands):


	December 31, 2023		December 31, 2022
United States	$	5,786,937	$	5,595,255
Canada and other foreign	595,932		534,452

Total	$	6,382,869	$	6,129,707

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
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-Name Accounting Standards Codification
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-22

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 14: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Accrued insurance	$ 107,658	$ 92,909
Accrued interest	33,857	20,033
Accrued compensation and benefits	113,236	123,226
Accrued income, real estate, sales and other taxes	44,752	61,442
Accrued other	97,654	99,106
Total accrued expenses	$ 397,157	$ 396,716

STOCKHOLDERS' EQUITY	12 Months Ended
STOCKHOLDERS' EQUITY	Dec. 31, 2023
Equity [Abstract]
STOCKHOLDERS' EQUITY	STOCKHOLDERS' EQUITY On December 5, 2023, the Board of Directors of Clean Harbors authorized a $500.0 million expansion of the Company’s current share repurchase program resulting in $554.1 million available at December 31, 2023. The repurchase program authorizes the Company to purchase the Company's common stock on the open market or in privately negotiated transactions periodically in a manner that complies with applicable U.S. securities laws. The number of shares purchased and the timing of the purchases have depended and will depend on a number of factors including share price, cash required for future business plans, trading volume and other conditions. The Company has no obligation to repurchase stock under this program and may suspend or terminate the repurchase program at any time. During the years ended December 31, 2023, 2022 and 2021, the Company repurchased and retired a total of 0.3 million, 0.5 million and 0.6 million shares, respectively, of the Company's common stock for total costs of $51.2 million, $50.2 million and $54.4 million, respectively.

- Definition

The entire disclosure for equity.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-URI https://asc.fasb.org//1943274/2147481112/505-10-50-16

Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

+ Details

FINANCING ARRANGEMENTS - Summary of Long-term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Current portion of long-term debt	$ 10,000	$ 10,000
Long-term debt, at par	2,315,000	2,438,975
Unamortized debt issuance costs and discount, net	(23,283)	(24,147)
Long-term debt, at carrying value	2,291,717	2,414,828
Secured debt \| Secured senior term loans due October 8, 2028 ("2028 Term Loans")
Debt Instrument [Line Items]
Current portion of long-term debt	10,000	10,000
Long-term debt, at par	970,000	980,000
Secured debt \| Secured senior term loans due June 30, 2024 ("2024 Term Loans")
Debt Instrument [Line Items]
Long-term debt, at par	0	613,975
Unsecured debt \| Unsecured senior notes, at 4.875%, due July 15, 2027 ("2027 Notes")
Debt Instrument [Line Items]
Long-term debt, at par	$ 545,000	545,000
Interest rate (as a percent)	4.875%
Unsecured debt \| Unsecured senior notes, at 5.125%, due July 15, 2029 ("2029 Notes")
Debt Instrument [Line Items]
Long-term debt, at par	$ 300,000	300,000
Interest rate (as a percent)	5.125%
Unsecured debt \| Unsecured senior notes, at 6.375%, due February 1, 2031 ("2031 Notes")
Debt Instrument [Line Items]
Long-term debt, at par	$ 500,000	$ 0
Interest rate (as a percent)	6.375%

SIGNIFICANT ACCOUNTING POLICIES - Cash, Cash Equivalents and Uncashed Checks (Details)	Dec. 31, 2023 USD ($)
Accounting Policies [Abstract]
Bank disbursement account balance	$ 0

FINANCING ARRANGEMENTS (Tables)

12 Months Ended

Dec. 31, 2023

Debt Disclosure [Abstract]

Schedule of Long-term Debt

The following table is a summary of the Company's long-term debt (in thousands):


	December 31, 2023		December 31, 2022
Current Portion of Long-Term Debt:
Secured senior term loans	$	10,000	$	10,000


Long-Term Debt:
Secured senior term loans due June 30, 2024 ("2024 Term Loans")	$	—	$	613,975
Secured senior term loans due October 8, 2028 ("2028 Term Loans")	970,000		980,000
Unsecured senior notes, at 4.875%, due July 15, 2027 ("2027 Notes")	545,000		545,000
Unsecured senior notes, at 5.125%, due July 15, 2029 ("2029 Notes")	300,000		300,000
Unsecured senior notes, at 6.375%, due February 1, 2031 ("2031 Notes")	500,000		—
Long-term debt, at par	2,315,000		2,438,975
Unamortized debt issuance costs and discount, net	(23,283)		(24,147)
Long-term debt, at carrying value	$	2,291,717	$	2,414,828

Schedule of Redemption Prices

2027 Notes


	Percentage

July 15, 2023	101.219	%
July 15, 2024 and thereafter	100.000	%

2029 Notes


	Percentage
July 15, 2024	102.563	%
July 15, 2025	101.281	%
July 15, 2026 and thereafter	100.000	%

2031 Notes


	Percentage
February 1, 2026	103.188	%
February 1, 2027	101.594	%
February 1, 2028 and thereafter	100.000	%

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 69B
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481568/470-20-55-69B

Reference 2: http://www.xbrl.org/2003/role/exampleRef
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-Name Accounting Standards Codification
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-URI https://asc.fasb.org//1943274/2147481568/470-20-55-69E

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
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-Name Accounting Standards Codification
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-Paragraph 3
-Subparagraph (SX 210.12-04(a))
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-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-3

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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+ Details

INVENTORIES AND SUPPLIES (Tables)

12 Months Ended

Dec. 31, 2023

Inventory Disclosure [Abstract]

Schedule of Inventories

Inventories and supplies consisted of the following (in thousands):


	December 31, 2023		December 31, 2022
Oil and oil related products	$	118,600	$	151,519
Supplies	177,217		143,743
Solvent and solutions	11,795		11,994
Other	19,899		17,738
Total inventories and supplies	$	327,511	$	324,994

STOCKHOLDERS' EQUITY (Details) - USD ($) shares in Millions	12 Months Ended
STOCKHOLDERS' EQUITY (Details) - USD ($) shares in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 05, 2023
Equity [Abstract]
Stock repurchase program, authorized amount				$ 500,000,000
Stock repurchase program, remaining authorized repurchase amount	$ 554,100,000
Repurchase of common stock (in shares)	0.3	0.5	0.6
Repurchases of common stock, excluding exercise tax	$ 51,200,000	$ 50,200,000	$ 54,400,000

SIGNIFICANT ACCOUNTING POLICIES - Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Marketable Securities [Line Items]
Total cash equivalents	$ 27,542	$ 34,008
Total marketable securities	106,101	62,033
Total	133,643	96,041
Municipal bonds
Marketable Securities [Line Items]
Total marketable securities	0	1,930
Commercial paper
Marketable Securities [Line Items]
Total marketable securities	56,172	24,075
Corporate notes and bonds
Marketable Securities [Line Items]
Total marketable securities	49,929	36,028
Commercial paper
Marketable Securities [Line Items]
Total cash equivalents	27,542	5,035
U.S. Treasury securities
Marketable Securities [Line Items]
Total cash equivalents	$ 0	$ 28,973

ACCUMULATED OTHER COMPREHENSIVE LOSS

12 Months Ended

Dec. 31, 2023

Equity [Abstract]

ACCUMULATED OTHER COMPREHENSIVE LOSS

The changes in accumulated other comprehensive loss by component and related tax impacts for the years ended December 31, 2023, 2022 and 2021 were as follows (in thousands):


	Foreign Currency Translation Adjustments		Unrealized Gain (Loss) on Available-For-Sale Securities		Unrealized (Loss) Gain on Fair Value of Interest Rate Hedges		Unrealized Loss on Pension		Total
Balance at January 1, 2021	$	(176,234)	$	135	$	(33,629)	$	(1,749)	$	(211,477)
Other comprehensive (loss) income before reclassifications	(1,590)		(361)		6,235		1,411		5,695
Amounts reclassified out of accumulated other comprehensive loss	—		—		10,011		—		10,011
Tax benefit (provision)	—		76		—		(317)		(241)
Other comprehensive (loss) income	(1,590)		(285)		16,246		1,094		15,465
Balance at December 31, 2021	(177,824)		(150)		(17,383)		(655)		(196,012)
Other comprehensive (loss) income before reclassifications	(31,515)		(523)		78,711		415		47,088
Amounts reclassified out of accumulated other comprehensive loss	—		—		(683)		—		(683)
Tax benefit (provision)	—		110		(17,587)		(97)		(17,574)
Other comprehensive (loss) income	(31,515)		(413)		60,441		318		28,831
Balance at December 31, 2022	(209,339)		(563)		43,058		(337)		(167,181)
Other comprehensive income (loss) before reclassifications	9,000		708		5,094		(741)		14,061
Amounts reclassified out of accumulated other comprehensive loss	—		—		(30,273)		—		(30,273)
Tax (provision) benefit	—		(149)		8,012		191		8,054
Other comprehensive income (loss)	9,000		559		(17,167)		(550)		(8,158)
Balance at December 31, 2023	$	(200,339)	$	(4)	$	25,891	$	(887)	$	(175,339)

The amounts realized in the consolidated statements of operations during the years ended December 31, 2023, 2022 and 2021 which were reclassified out of accumulated other comprehensive loss were as follows (in thousands):


	For the years ended December 31,
Other Comprehensive Loss Component	2023		2022		2021		Location

Unrealized (Loss) Gain on Fair Value of Interest Rate Hedges (1)	$	30,273	$	683	$	(10,011)	Interest expense, net of interest income

___________________________________

(1) For the year ended December 31, 2023, the balance is inclusive of an $8.3 million gain realized in connection with the settlement of the 2018 Swaps. For more information on this transaction, see Note 12, "Financing Arrangements."

ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Accrued insurance, employee medical insurance costs	$ 22.8	$ 16.4
Accrual loss for workers' compensation, comprehensive general liability and vehicle liability self-insurance programs	$ 82.7	$ 74.9

LEASES (Tables)

12 Months Ended

Dec. 31, 2023

Leases [Abstract]

Schedule of Supplemental Lease Balance Sheet Information

The following table presents the Company's finance lease balances and their classification on the consolidated balance sheets (in thousands):


Finance Lease Balances (Classification)	December 31, 2023		December 31, 2022
ROU assets (Property, plant and equipment, net)	$	126,292	$	96,207
Current portion of lease liabilities (Accrued expenses and other current liabilities)	16,975		12,767
Long-term portion of lease liabilities (Other long-term liabilities)	113,486		86,390

Schedule of Lease Expense and Other Information

The Company’s lease expense was as follows (in thousands):


	For the years ended December 31,
	2023		2022		2021
Operating lease cost	$	69,156	$	59,041	$	50,264

Finance lease cost:
Amortization of ROU assets	17,183		13,239		9,504
Interest on lease liabilities	4,051		3,011		2,544
Total finance lease cost	21,234		16,250		12,048

Short-term lease cost	167,379		158,736		102,913
Variable lease cost	4,887		4,353		3,546
Total lease cost	$	262,656	$	238,380	$	168,771

Other information related to leases was as follows:


Weighted Average Remaining Lease Term (years)	December 31, 2023		December 31, 2022
Operating leases	3.8		4.2
Finance leases	7.1		7.5

Weighted Average Discount Rate	December 31, 2023		December 31, 2022
Operating leases	4.68	%	4.08	%
Finance leases	3.96	%	3.53	%


	For the years ended December 31,
Supplemental Cash Flow Related Disclosures (in thousands)	2023		2022		2021
Cash paid for amounts related to lease liabilities:
Operating cash flows from operating leases	$	71,185	$	61,014	$	50,963
Operating cash flows from finance leases	4,051		3,011		2,544
Financing cash flows from finance leases	15,937		12,821		8,458
ROU assets obtained in exchange for operating lease liabilities	82,392		58,230		55,556
ROU assets obtained in exchange for finance lease liabilities	47,238		29,205		30,476

Schedule of Operating Lease Maturity

At December 31, 2023, the Company's future lease payments under non-cancelable leases that have lease terms in excess of one year were as follows (in thousands):


Years ending December 31,	December 31, 2023
	Operating Leases		Finance Leases
2024	$	66,372	$	23,703
2025	53,215		23,017
2026	38,927		23,029
2027	28,011		23,016
2028	12,925		19,040
Thereafter	9,408		41,681
Total future lease payments	208,858		153,486
Amount representing interest	(20,685)		(23,025)
Total lease liabilities	$	188,173	$	130,461

Schedule of Finance Lease Maturity

At December 31, 2023, the Company's future lease payments under non-cancelable leases that have lease terms in excess of one year were as follows (in thousands):


Years ending December 31,	December 31, 2023
	Operating Leases		Finance Leases
2024	$	66,372	$	23,703
2025	53,215		23,017
2026	38,927		23,029
2027	28,011		23,016
2028	12,925		19,040
Thereafter	9,408		41,681
Total future lease payments	208,858		153,486
Amount representing interest	(20,685)		(23,025)
Total lease liabilities	$	188,173	$	130,461

GOODWILL AND OTHER INTANGIBLE ASSETS - Expected Amortization (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Expected amortization
2024	$ 46,639
2025	43,231
2026	41,385
2027	39,245
2028	37,975
Thereafter	274,037
Finite-lived intangible assets, net	$ 482,512	$ 498,248

LEASES - Schedule of Future Lease Payments (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Operating Leases
2024	$ 66,372
2025	53,215
2026	38,927
2027	28,011
2028	12,925
Thereafter	9,408
Total future lease payments	208,858
Amount representing interest	(20,685)
Total lease liabilities	188,173
Finance Leases
2024	23,703
2025	23,017
2026	23,029
2027	23,016
2028	19,040
Thereafter	41,681
Total future lease payments	153,486
Amount representing interest	(23,025)
Total lease liabilities	$ 130,461

STOCK-BASED COMPENSATION - Performance Stock Awards (Details) - Performance Stock Awards - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Number of Shares
Unvested, beginning of period (in shares)	213,679
Granted (in shares)	120,765
Vested (in shares)	(111,924)
Forfeited (in shares)	(41,236)
Unvested, end of period (in shares)	181,284	213,679
Weighted Average Grant-Date Fair Value
Unvested, beginning of period (in dollars per share)	$ 91.62
Granted (in dollars per share)	131.18
Vested (in dollars per share)	90.56
Forfeited (in dollars per share)	111.50
Unvested, end of period (in dollars per share)	$ 114.10	$ 91.62
Unrecognized compensation cost	$ 3.3
Fair value of share-based payment awards	$ 17.0	$ 11.1	$ 15.0

LEASES

12 Months Ended

Dec. 31, 2023

Leases [Abstract]

LEASES

The Company’s lease portfolio is predominately operating leases for real estate, vehicles and industrial equipment utilized in operations and rail cars. The Company presents operating lease balances separately on the consolidated balance sheets. The Company's finance leases relate to vehicles, rail cars and certain real estate. The following table presents the Company's finance lease balances and their classification on the consolidated balance sheets (in thousands):


Finance Lease Balances (Classification)	December 31, 2023		December 31, 2022
ROU assets (Property, plant and equipment, net)	$	126,292	$	96,207
Current portion of lease liabilities (Accrued expenses and other current liabilities)	16,975		12,767
Long-term portion of lease liabilities (Other long-term liabilities)	113,486		86,390

The Company’s lease expense was as follows (in thousands):


	For the years ended December 31,
	2023		2022		2021
Operating lease cost	$	69,156	$	59,041	$	50,264

Finance lease cost:
Amortization of ROU assets	17,183		13,239		9,504
Interest on lease liabilities	4,051		3,011		2,544
Total finance lease cost	21,234		16,250		12,048

Short-term lease cost	167,379		158,736		102,913
Variable lease cost	4,887		4,353		3,546
Total lease cost	$	262,656	$	238,380	$	168,771

Other information related to leases was as follows:


Weighted Average Remaining Lease Term (years)	December 31, 2023		December 31, 2022
Operating leases	3.8		4.2
Finance leases	7.1		7.5

Weighted Average Discount Rate	December 31, 2023		December 31, 2022
Operating leases	4.68	%	4.08	%
Finance leases	3.96	%	3.53	%


	For the years ended December 31,
Supplemental Cash Flow Related Disclosures (in thousands)	2023		2022		2021
Cash paid for amounts related to lease liabilities:
Operating cash flows from operating leases	$	71,185	$	61,014	$	50,963
Operating cash flows from finance leases	4,051		3,011		2,544
Financing cash flows from finance leases	15,937		12,821		8,458
ROU assets obtained in exchange for operating lease liabilities	82,392		58,230		55,556
ROU assets obtained in exchange for finance lease liabilities	47,238		29,205		30,476

At December 31, 2023, the Company's future lease payments under non-cancelable leases that have lease terms in excess of one year were as follows (in thousands):


Years ending December 31,	December 31, 2023
	Operating Leases		Finance Leases
2024	$	66,372	$	23,703
2025	53,215		23,017
2026	38,927		23,029
2027	28,011		23,016
2028	12,925		19,040
Thereafter	9,408		41,681
Total future lease payments	208,858		153,486
Amount representing interest	(20,685)		(23,025)
Total lease liabilities	$	188,173	$	130,461

At December 31, 2023, none of the Company's executed leases that had not yet commenced will create significant rights or obligations in the future and sublease transactions are not material. Additionally, the Company does not have any related party leases, and there were no restrictions or covenants imposed by its leases.

LEASES


Finance Lease Balances (Classification)	December 31, 2023		December 31, 2022
ROU assets (Property, plant and equipment, net)	$	126,292	$	96,207
Current portion of lease liabilities (Accrued expenses and other current liabilities)	16,975		12,767
Long-term portion of lease liabilities (Other long-term liabilities)	113,486		86,390

The Company’s lease expense was as follows (in thousands):


	For the years ended December 31,
	2023		2022		2021
Operating lease cost	$	69,156	$	59,041	$	50,264

Finance lease cost:
Amortization of ROU assets	17,183		13,239		9,504
Interest on lease liabilities	4,051		3,011		2,544
Total finance lease cost	21,234		16,250		12,048

Short-term lease cost	167,379		158,736		102,913
Variable lease cost	4,887		4,353		3,546
Total lease cost	$	262,656	$	238,380	$	168,771

Other information related to leases was as follows:


Weighted Average Remaining Lease Term (years)	December 31, 2023		December 31, 2022
Operating leases	3.8		4.2
Finance leases	7.1		7.5

Weighted Average Discount Rate	December 31, 2023		December 31, 2022
Operating leases	4.68	%	4.08	%
Finance leases	3.96	%	3.53	%


	For the years ended December 31,
Supplemental Cash Flow Related Disclosures (in thousands)	2023		2022		2021
Cash paid for amounts related to lease liabilities:
Operating cash flows from operating leases	$	71,185	$	61,014	$	50,963
Operating cash flows from finance leases	4,051		3,011		2,544
Financing cash flows from finance leases	15,937		12,821		8,458
ROU assets obtained in exchange for operating lease liabilities	82,392		58,230		55,556
ROU assets obtained in exchange for finance lease liabilities	47,238		29,205		30,476

At December 31, 2023, the Company's future lease payments under non-cancelable leases that have lease terms in excess of one year were as follows (in thousands):


Years ending December 31,	December 31, 2023
	Operating Leases		Finance Leases
2024	$	66,372	$	23,703
2025	53,215		23,017
2026	38,927		23,029
2027	28,011		23,016
2028	12,925		19,040
Thereafter	9,408		41,681
Total future lease payments	208,858		153,486
Amount representing interest	(20,685)		(23,025)
Total lease liabilities	$	188,173	$	130,461

REVENUES - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Total revenues	$ 5,409,152	$ 5,166,605	$ 3,805,566
Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	1,563,847	1,495,264	1,209,624
Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	1,419,385	1,311,579	706,298
Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	609,913	575,300	466,380
Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	1,102,041	962,697	805,491
Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	713,966	821,765	617,773
United States
Disaggregation of Revenue [Line Items]
Total revenues	4,869,180	4,576,167	3,324,953
Canada
Disaggregation of Revenue [Line Items]
Total revenues	539,972	590,438	480,613
Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	4,511,442	4,171,706	3,032,454
Safety-Kleen Sustainability Solutions
Disaggregation of Revenue [Line Items]
Total revenues	897,263	994,392	772,813
Operating segments \| Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	4,469,909	4,144,973	3,025,907
Operating segments \| Environmental Services \| Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	1,563,847	1,495,264	1,209,624
Operating segments \| Environmental Services \| Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	1,418,938	1,311,072	705,999
Operating segments \| Environmental Services \| Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	609,913	575,300	466,380
Operating segments \| Environmental Services \| Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	877,211	763,337	643,904
Operating segments \| Environmental Services \| Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Operating segments \| Environmental Services \| United States
Disaggregation of Revenue [Line Items]
Total revenues	4,022,394	3,675,880	2,631,112
Operating segments \| Environmental Services \| Canada
Disaggregation of Revenue [Line Items]
Total revenues	447,515	469,093	394,795
Operating segments \| Safety-Kleen Sustainability Solutions
Disaggregation of Revenue [Line Items]
Total revenues	938,796	1,021,125	779,360
Operating segments \| Safety-Kleen Sustainability Solutions \| Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Operating segments \| Safety-Kleen Sustainability Solutions \| Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Operating segments \| Safety-Kleen Sustainability Solutions \| Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Operating segments \| Safety-Kleen Sustainability Solutions \| Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	224,830	199,360	161,587
Operating segments \| Safety-Kleen Sustainability Solutions \| Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	713,966	821,765	617,773
Operating segments \| Safety-Kleen Sustainability Solutions \| United States
Disaggregation of Revenue [Line Items]
Total revenues	846,339	899,780	693,542
Operating segments \| Safety-Kleen Sustainability Solutions \| Canada
Disaggregation of Revenue [Line Items]
Total revenues	92,457	121,345	85,818
Corporate, non-segment
Disaggregation of Revenue [Line Items]
Total revenues	447	507	299
Corporate, non-segment \| Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Corporate, non-segment \| Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	447	507	299
Corporate, non-segment \| Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Corporate, non-segment \| Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Corporate, non-segment \| Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Corporate, non-segment \| United States
Disaggregation of Revenue [Line Items]
Total revenues	447	507	299
Corporate, non-segment \| Canada
Disaggregation of Revenue [Line Items]
Total revenues	$ 0	$ 0	$ 0

REMEDIAL LIABILITIES - Anticipated Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accrual for Environmental Loss Contingencies, Net [Abstract]
2024	$ 13,898
2025	19,253
2026	9,006
2027	7,001
2028	7,767
Thereafter	71,737
Undiscounted remedial liabilities	128,662
Less: Discount at risk free rates	(17,419)
Total remedial liabilities	$ 111,243	$ 116,290	$ 111,873
Environmental Loss Contingency, Statement of Financial Position [Extensible Enumeration]	Remedial liabilities, less current portion of $13,358 and $9,918, respectively, Current portion of closure, post-closure and remedial liabilities

REMEDIAL LIABILITIES - Changes in Remedial Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Site Contingency [Line Items]
Environmental Loss Contingency, Current, Statement of Financial Position [Extensible Enumeration]	Current portion of closure, post-closure and remedial liabilities	Current portion of closure, post-closure and remedial liabilities
Changes to remedial liabilities
Balance at the beginning of the period	$ 116,290	$ 111,873
Liabilities assumed in acquisition of real estate		8,092
Measurement period adjustments from prior period acquisition		338
Accretion	4,190	4,191
Changes in estimates recorded to consolidated statement of operations	3,628	1,497
Environmental expenditures	(13,276)	(8,795)
Currency translation and other	411	(906)
Balance at the end of the period	111,243	116,290
Remedial Liabilities for Landfill Sites
Changes to remedial liabilities
Balance at the beginning of the period	1,824	1,780
Liabilities assumed in acquisition of real estate		0
Measurement period adjustments from prior period acquisition		0
Accretion	89	86
Changes in estimates recorded to consolidated statement of operations	19	8
Environmental expenditures	(52)	(50)
Currency translation and other	0	0
Balance at the end of the period	1,880	1,824
Remedial Liabilities for Inactive Sites
Changes to remedial liabilities
Balance at the beginning of the period	59,749	59,787
Liabilities assumed in acquisition of real estate		0
Measurement period adjustments from prior period acquisition		0
Accretion	2,540	2,498
Changes in estimates recorded to consolidated statement of operations	1,713	877
Environmental expenditures	(3,840)	(3,370)
Currency translation and other	115	(43)
Balance at the end of the period	60,277	59,749
Remedial Liabilities (Including Superfund) for Non-Landfill Operations
Changes to remedial liabilities
Balance at the beginning of the period	54,717	50,306
Liabilities assumed in acquisition of real estate		8,092
Measurement period adjustments from prior period acquisition		338
Accretion	1,561	1,607
Changes in estimates recorded to consolidated statement of operations	1,896	612
Environmental expenditures	(9,384)	(5,375)
Currency translation and other	296	(863)
Balance at the end of the period	$ 49,086	$ 54,717

FINANCING ARRANGEMENTS - Additional Information (Details) - USD ($)						1 Months Ended	6 Months Ended	12 Months Ended
	Dec. 31, 2023	Dec. 27, 2023	Apr. 28, 2023	Jan. 24, 2023	Oct. 28, 2020	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jul. 01, 2023	Jul. 02, 2019
Financing arrangements
Fair value of long-term obligations, including current portion	$ 2,300,000,000					$ 2,300,000,000	$ 2,300,000,000	$ 2,300,000,000	$ 2,400,000,000
Loss on early extinguishment of debt								(2,880,000)	(422,000)	$ 0
Revolving credit facility, available borrowing capacity	265,700,000					265,700,000	265,700,000	265,700,000
Letters of credit outstanding, amount	134,300,000					134,300,000	134,300,000	134,300,000
Borrowing from revolving credit facility								114,000,000	$ 0	0
2018 Swaps
Financing arrangements
Reclassification of gain from accumulated other comprehensive loss to interest expense				$ 8,300,000				8,300,000
Derivative notional amount	350,000,000					350,000,000	350,000,000	350,000,000
2022 Swaps \| Through June 30, 2023
Financing arrangements
Derivative, fixed interest rate									0.931%
2022 Swaps \| From July 1, 2023 through September 30, 2027
Financing arrangements
Derivative, fixed interest rate											1.9645%
Interest rate swap
Financing arrangements
Reclassification of gain from accumulated other comprehensive loss to interest expense								30,273,000	$ 683,000	$ (10,011,000)
Derivative asset	35,500,000					35,500,000	35,500,000	35,500,000	60,600,000
Secured senior term loans due October 8, 2028 ("2028 Term Loans") \| 2022 Swaps
Financing arrangements
Derivative notional amount									600,000,000
Secured debt \| Secured senior term loans due October 8, 2028 ("2028 Term Loans")
Financing arrangements
Debt outstanding	980,000,000					$ 980,000,000	$ 980,000,000	980,000,000
Decrease of interest rate margin		0.25%
Debt, soft call period						6 months
Aggregate principal amount									$ 600,000,000
Effective interest rate (as a percent)		3.82898%									4.07898%
Secured debt \| Secured senior term loans due October 8, 2028 ("2028 Term Loans") \| 2022 Swaps
Financing arrangements
Effective interest rate (as a percent)									2.931%
Secured debt \| Secured senior term loans due October 8, 2028 ("2028 Term Loans") \| Base rate
Financing arrangements
Basis spread on variable rate (as a percent)		0.75%
Debt, floor interest rate		1.00%
Secured debt \| Secured senior term loans due October 8, 2028 ("2028 Term Loans") \| Secured Overnight Financing Rate (SOFR)
Financing arrangements
Basis spread on variable rate (as a percent)		0.11448%					0.11448%
Debt, interest margin		1.75%							2.00%
Debt, floor interest rate		0.00%
Secured debt \| Secured senior term loans due June 30, 2024 ("2024 Term Loans")
Financing arrangements
Repayments of debt				614,000,000
Secured debt \| Secured senior term loans due June 30, 2024 ("2024 Term Loans") \| 2018 Swaps
Financing arrangements
Debt outstanding	350,000,000					$ 350,000,000	$ 350,000,000	350,000,000
LIne of credit
Financing arrangements
Credit facility, maximum borrowing capacity			$ 400,000,000
Period for measurement of average liquidity (in days)			30 days
LIne of credit \| Minimum
Financing arrangements
Commitment fee per annum of the unused commitment			0.25%
LIne of credit \| Maximum
Financing arrangements
Commitment fee per annum of the unused commitment			0.375%
LIne of credit \| Parent and Domestic Subsidiaries
Financing arrangements
Credit available subject to percentage of accounts receivable (as a percent)					85.00%
Credit available subject to percentage of cash deposited (as a percent)					100.00%
LIne of credit \| Canadian Subsidiaries
Financing arrangements
Credit available subject to percentage of accounts receivable (as a percent)					85.00%
Credit available subject to percentage of cash deposited (as a percent)					100.00%
LIne of credit \| Revolving credit facility
Financing arrangements
Debt outstanding	$ 0					$ 0	$ 0	$ 0	$ 0
Net proceeds from the company’s issuance				114,000,000
Borrowing from revolving credit facility				114,000,000
LIne of credit \| Revolving credit facility \| Parent and Domestic Subsidiaries
Financing arrangements
Credit facility, maximum borrowing capacity					$ 350,000,000
LIne of credit \| Revolving credit facility \| Canadian Subsidiaries
Financing arrangements
Credit facility, maximum borrowing capacity					50,000,000
LIne of credit \| Letters of credit \| Parent and Domestic Subsidiaries
Financing arrangements
Credit facility, maximum borrowing capacity					$ 250,000,000
LIne of credit \| Base rate \| Minimum
Financing arrangements
Basis spread on variable rate (as a percent)			0.50%
LIne of credit \| Base rate \| Maximum
Financing arrangements
Basis spread on variable rate (as a percent)			0.75%
LIne of credit \| Secured Overnight Financing Rate (SOFR) \| Minimum
Financing arrangements
Basis spread on variable rate (as a percent)			1.50%
LIne of credit \| Secured Overnight Financing Rate (SOFR) \| Maximum
Financing arrangements
Basis spread on variable rate (as a percent)			1.75%
LIne of credit \| Secured senior term loans due October 8, 2028 ("2028 Term Loans") \| Secured Overnight Financing Rate (SOFR)
Financing arrangements
Basis spread on variable rate (as a percent)	1.86448%
Unsecured debt
Financing arrangements
Redemption percentage of principal amount								100.00%
Unsecured debt \| Unsecured senior notes, at 4.875%, due July 15, 2027 ("2027 Notes")
Financing arrangements
Aggregate principal amount												$ 545,000,000
Unsecured debt \| Unsecured senior notes, at 5.125%, due July 15, 2029 ("2029 Notes")
Financing arrangements
Aggregate principal amount												$ 300,000,000
Unsecured debt \| Unsecured senior notes, at 6.375%, due February 1, 2031 ("2031 Notes")
Financing arrangements
Net proceeds from the company’s issuance				500,000,000
Aggregate principal amount				$ 500,000,000
Redemption percentage of principal amount								106.375%
Debt instrument, redemption price, percentage of principal amount redeemed								40.00%

SIGNIFICANT ACCOUNTING POLICIES (Policies)

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Principles of Consolidation

The accompanying consolidated financial statements include the accounts of Clean Harbors, Inc. and its majority-owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation.

Use of Estimates

The preparation of consolidated financial statements in conformity with U.S. Generally Accepted Accounting Principles (GAAP), requires management to make estimates and assumptions about future events that affect the amounts reported in the consolidated financial statements and the accompanying notes. Management bases its estimates on historical experience and other assumptions that it believes to be reasonable at the time. Actual results could differ from those estimates, and any such differences may be material to the consolidated financial statements. Estimates and assumptions are reviewed periodically and the effects of changes, if any, are reflected in our consolidated financial statements in the period they are determined.

Cash, Cash Equivalents, Marketable Securities and Uncashed Checks

Cash consists primarily of cash on deposit and money market accounts. The Company, through its wholly-owned captive insurance subsidiary, invests in marketable securities. Marketable securities with maturities of three months or less from the date of purchase are classified as cash equivalents.

Cash, Cash Equivalents, Marketable Securities and Uncashed Checks

Realized gains and losses on sales of available-for-sale marketable securities in the years presented were immaterial. The majority of the marketable securities have a remaining maturity of less than one year and fair value approximates cost.

The Company's cash management program with its revolving credit lender allows the Company to maintain a zero balance in the U.S. bank disbursement accounts that are used to issue vendor and payroll checks. When checks are presented to the bank for payment, cash deposits in amounts sufficient to fund the checks are made, at the Company's discretion, either from funds provided by other accounts or under the terms of the Company's revolving credit facility. Checks that have been written to vendors or employees but have not yet been presented for payment at the Company's bank are classified as uncashed checks in accounts payable and changes in the balance are reported as a financing activity in the consolidated statements of cash flows.

Allowances for Doubtful Accounts

Allowance for Doubtful Accounts and Revenue Allowance

On a regular basis, the Company evaluates its accounts receivable and establishes the allowance for doubtful accounts based on an evaluation of certain criteria and evidence of collection uncertainty including historical collection trends, reasonable expectations of future collections, current economic trends and changes in customer payment patterns. Past-due receivable balances are written off when the Company's collection efforts have been deemed unsuccessful in collecting the outstanding balance due.

Due to the nature of the Company's businesses and the invoices that result from the services provided, customers may withhold payments and attempt to renegotiate amounts invoiced. In addition, for some of the services provided, the Company's invoices are based on quotes that, in limited instances, can result in adjustments to revenue subsequent to billing. Based on industry knowledge and historical trends, the Company records a revenue allowance in anticipation of these expected adjustments. This practice causes the volume of activity flowing through the revenue allowance during the year to be higher than the balance at the end of the year. The revenue allowance is intended to cover the net amount of revenue adjustments that may need to be credited to customers' accounts in future periods. Management determines the appropriate total revenue allowance by evaluating the following factors on an invoice-by-invoice basis as well as on a consolidated level: trends in adjustments to previously billed amounts, existing economic conditions, communications with customers and other information as deemed applicable. Revenue allowance estimates can differ from the actual adjustments, but historically the revenue allowance has been sufficient to cover the net amount of the reserve adjustments issued in subsequent reporting periods.

Revenue Allowance

Allowance for Doubtful Accounts and Revenue Allowance

Credit Concentration

Concentration of credit risks in accounts receivable is limited due to the large number of customers comprising the Company's customer base throughout North America. The Company maintains policies over credit extension that include credit evaluations, credit limits and collection monitoring procedures on a customer-by-customer basis. However, the Company generally does not require collateral before services are performed. No individual customer accounted for more than 10% of accounts receivable or more than 10% of total third-party revenues in the periods presented.

Inventories and Supplies

Inventories are stated at the lower of cost or market. The cost of oil and oil products as well as the cost of supplies and drums, solvent and solution and other inventories is principally determined on a first-in, first-out ("FIFO") basis. The Company continually reviews its inventories for obsolete or unsalable items and adjusts its carrying value to reflect estimated realizable values.

Property, Plant and Equipment, net (excluding landfill assets and finance lease right-of-use assets)

Property, plant and equipment, net is stated at cost less accumulated depreciation. Expenditures for major renewals and improvements which extend the life or usefulness of the asset are capitalized. Items of an ordinary repair or maintenance nature are charged directly to operating expense as incurred. During the construction and development period of an asset, the costs incurred, including interest expense, are classified as construction-in-progress. When the asset is ready for its intended use, the asset is reclassified to an appropriate asset classification and depreciation or amortization commences.

The Company depreciates and amortizes the capitalized cost of these assets, using the straight-line method as follows:


Asset Classification						Estimated Useful Life
Buildings and building improvements
Buildings						20-42 years
Leasehold and building improvements						2-45 years
Vehicles						2-15 years
Equipment
Capitalized software and computer equipment						3-5 years
Containers and railcars						8-16 years
All other equipment						4-30 years
Furniture and fixtures						5-8 years

Gains and losses on the sale of property, plant and equipment are included in Other income (expense), net. Fully depreciated assets are retained in property, plant and equipment and accumulated depreciation until they are removed from service.

The Company tests asset groups for impairment whenever events or changes in circumstances indicate that the carrying value may not be recoverable. An impairment in the carrying value of long-lived assets is recognized if the expected future undiscounted cash flows derived from the assets, or group of assets, are less than their carrying value.

Business Combinations

In accordance with the acquisition method of accounting, the purchase price paid for an acquisition is allocated to the assets and liabilities acquired based upon their estimated fair values as of the acquisition date, with the excess of the purchase price over the net assets acquired recorded as goodwill. As required, a preliminary fair value is determined once a business is acquired, with the final determination of the fair value being completed no later than one year from the date of acquisition.

Goodwill

Goodwill is comprised of the purchase price of business acquisitions in excess of the fair value of the net assets acquired. Goodwill is reviewed for impairment annually as of December 31 or when events or changes in the business environment indicate the carrying value of a reporting unit may exceed its fair value. This review is performed by comparing the fair value of each reporting unit to its carrying value, including goodwill. If the fair value is less than the carrying amount, a loss is recorded for the excess of the carrying value over the fair value up to the carrying amount of goodwill.

The Company determines its reporting units by identifying the components of each operating segment. As of December 31, 2023, the Company had four reporting units consisting of, Environmental Sales and Service, Industrial Services, Environmental Facilities and Safety-Kleen Sustainability Solutions ("SKSS").

Permits and Other Intangibles

Costs related to acquiring licenses, permits and intangible assets, such as legal fees, site surveys, engineering costs and other expenditures are capitalized. Other intangible assets consist primarily of customer and supplier relationships, trademarks and trade names and developed technology.

Permits relating to landfills are amortized on a units-of-consumption basis. All other permits are amortized over periods ranging from five to 30 years on a straight-line basis. Finite-lived other intangible assets are amortized on a straight-line basis over their respective useful lives, which range from two to 25 years.

All finite-lived intangible assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying value may not be recoverable. When such factors and circumstances exist, management compares the projected undiscounted future cash flows associated with the related asset or group of assets to the carrying amount. The impairment loss, if any, is measured as the excess of the carrying amount over the fair value of the asset or group of assets.

Indefinite-lived intangible assets are not amortized but are reviewed for impairment annually as of December 31, or when events or changes in the business environment indicate that the carrying value may be impaired. If the fair value of the asset is less than the carrying amount, the impairment loss is measured as the excess of the carrying value of the asset over its fair value.

Landfill Accounting

The Company amortizes landfill improvements and certain landfill-related permits over the estimated useful lives. The units-of-consumption method is used to amortize land, landfill cell construction, asset retirement costs and remaining landfill cells and sites. The Company also utilizes the units-of-consumption method to record closure and post-closure obligations for landfill cells and sites. Under the units-of-consumption method, the Company includes future estimated construction and asset retirement costs, as well as costs incurred to date, in the amortization base of the landfill assets. Additionally, where appropriate, as described below, the Company includes probable expansion airspace that has yet to be permitted in the calculation of the total remaining useful life of the landfill. If it is determined that expansion capacity should no longer be considered in calculating the recoverability of a landfill asset, the Company may be required to recognize an asset impairment or incur significantly higher amortization expense. If at any time the Company makes the decision to abandon the expansion effort, the capitalized costs related to the expansion effort are expensed immediately.

Landfill assets—Landfill assets include the costs of landfill site acquisition, permits and cell construction incurred to date. These amounts are recorded at cost, which includes capitalized interest as applicable. Landfill assets, net of amortization, are combined with management's estimate of the costs required to complete construction of the landfill to determine the amount to be amortized over the remaining estimated useful economic life of a site. Amortization of landfill assets is recorded on a units-of-consumption basis, such that the landfill assets should be completely amortized at the date the landfill ceases accepting waste. Amortization totaled $18.6 million, $16.2 million and $13.7 million for the years ended December 31, 2023, 2022 and 2021, respectively. Changes in the determination of when the landfill will cease accepting waste, either through a business decision by the Company, determination that expansion capacity should no longer be considered probable or changes in estimates on annual airspace consumption, will impact the amortization expense of the landfill assets. Changes in estimated costs to complete construction are applied prospectively to the amortization rate.

Landfill capacity—Landfill capacity, which is the basis for the amortization of landfill assets and for the accrual of final closure and post-closure obligations, represents total permitted airspace plus unpermitted airspace that management believes is highly probable of ultimately being permitted. As of December 31, 2023, there were no unpermitted expansions included in the Company's landfill accounting model. If actual expansion airspace is significantly different from management's estimate of expansion airspace, the amortization rates used for the units-of-consumption method would change, therefore impacting the Company's profitability.

As of December 31, 2023, the Company had eight active landfill sites (including the Company's non-commercial landfill), which have estimated remaining lives (based on anticipated waste volumes and the remaining highly probable airspace) as follows:


Facility Name	Location	Remaining Lives (Years)	Permitted Remaining Highly Probable Airspace (cubic yards) (in thousands)
Buttonwillow	California	20	4,999
Deer Trail	Colorado	21	1,420
Grassy Mountain	Utah	34	4,320
Kimball	Nebraska	23	547
Lambton	Ontario, Canada	46	4,350
Lone Mountain	Oklahoma	16	3,165
Ryley	Alberta, Canada	30	5,692
Sawyer	North Dakota	65	3,260
			27,753

In 2023, as expected, the Company's non-commercial landfill at Deer Park, Texas reached permitted capacity and is in the process of closing. This landfill has been excluded from the table above.

At December 31, 2023 and 2022, the Company had no cubic yards of permitted, but not highly probable, airspace.

The following table presents the remaining highly probable airspace from January 1, 2022 through December 31, 2023 (in thousands of cubic yards):


	2023	2022
Remaining capacity, beginning of year	28,270	23,784
Changes in highly probable airspace, net	464	5,579
Consumed	(981)	(1,093)
Remaining capacity, end of year	27,753	28,270

In the year ended December 31, 2023 the Company received a permit for expansion of its Kimball, Nebraska landfill. In the year ended December 31, 2022, the Company received a permit for the expansion of its Ryley landfill in Alberta, Canada.

Amortization of cell construction costs and accrual of cell closure obligations—Landfills are typically comprised of a number of cells, which are constructed within a defined acreage (or footprint). The cells are typically discrete units, which require both separate construction and separate capping and closure procedures. Cell construction costs are the costs required to excavate and construct the landfill cell. These costs are typically amortized on a units-of-consumption basis, such that they are completely amortized when the specific cell ceases accepting waste. In some instances, the Company has landfills that are engineered and constructed as "progressive trenches." In progressive trench landfills, a number of contiguous cells form a progressive trench. In those instances, the Company amortizes cell construction costs over the airspace within the entire trench, such that the cell construction costs will be fully amortized at the end of the trench useful life.

The design and construction of a landfill does not create a landfill asset retirement obligation. Rather, the asset retirement obligation for cell closure (the cost associated with capping each cell) is incurred in relatively small increments as waste is placed in the landfill. Therefore, the cost required to construct the cell cap is capitalized as an asset retirement cost and a liability of an equal amount is established, based on the discounted cash flow associated with each capping event, as airspace is consumed. Spending for cell capping is reflected as environmental expenditures within operating activities in the consolidated statements of cash flows.

Landfill final closure and post-closure liabilities —The balance of landfill final closure and post-closure liabilities at December 31, 2023 and 2022 was $59.4 million and $62.3 million, respectively. The Company has material financial commitments for the costs associated with requirements of the Environmental Protection Agency ("EPA") and the comparable regulatory agency in Canada for landfill final closure and post-closure activities. The Company develops estimates for the cost of these activities based on an evaluation of site-specific facts and circumstances, including the Company's interpretation of current regulatory requirements and proposed regulatory changes. Such estimates may change in the future due to various circumstances including, but not limited to, permit modifications, changes in legislation or regulations, technological changes and results of environmental studies.

Final closure costs are the costs incurred after the site ceases to accept waste, but before the landfill is certified as closed by the applicable state regulatory agency. These costs generally include the costs required to cap the final cell of the landfill (if not included in cell closure), the costs required to dismantle certain structures for landfills and other landfill improvements, and regulation-mandated groundwater monitoring and leachate management. Post-closure costs involve the maintenance and monitoring of a landfill site that has been certified closed by the applicable regulatory agency. These costs generally include groundwater monitoring and leachate management. Regulatory post-closure periods are generally 30 years after landfill closure. Final closure and post-closure obligations are accrued on a units-of-consumption basis, such that the present value of the final closure and post-closure obligations are fully accrued at the date the landfill ceases accepting waste.

Cell closure, final closure and post-closure costs (also referred to as "asset retirement obligations") are calculated by estimating the total obligation in current dollars, adjusted for future inflation estimates and discounted at the Company's credit-adjusted risk-free interest rate (6.51% and 5.37% during 2023 and 2022, respectively).

Non-Landfill Closure and Post-Closure Liabilities

The balance of non-landfill closure and post-closure liabilities at December 31, 2023 and 2022 was $59.2 million and $56.6 million, respectively. Non-landfill closure and post-closure obligations arise when the Company commences non-landfill facility operations and include costs required to dismantle and decontaminate certain structures and other costs incurred during the closure process. Post-closure costs, if required, include associated maintenance and monitoring costs as required by the closure permit. Post-closure periods are performance-based and are not typically specified in terms of years in the closure permit, but generally range from 10 to 30 years or more.

The Company records its non-landfill closure and post-closure liability by: (i) estimating the current cost of closing a non-landfill facility and the post-closure care of that facility, if required, based upon the closure plan that the Company is required to follow under its operating permit, or in the event the facility operates with a permit that does not contain a closure plan, based upon legally enforceable closure commitments made by the Company to various government agencies; (ii) estimates as to when future operations may cease; (iii) cost estimates of closing the non-landfill facility using the inflation rate to the time of closing; and (iv) discounting the future value back to the present using the credit-adjusted risk-free interest rate.

The estimates for non-landfill closure and post-closure liabilities are inherently uncertain due to the possibility that permit and regulatory requirements will change in the future, impacting the estimation of total costs and the timing of the expenditures. Management reviews non-landfill closure and post-closure liabilities for changes to key assumptions that would impact the amount of the recorded liabilities. Changes that would prompt management to revise a liability estimate include changes in legal requirements that impact the Company's expected closure plan or scope of work, in the market price of a significant cost item, in the estimate as to when future operations at a location might cease or in the expected timing of the costs. Changes in estimates for non-landfill closure and post-closure events immediately impact the liability and the value of the corresponding asset. If a change is made to a fully-amortized asset, the adjustment is charged immediately to expense. When a change in estimate relates to an asset that has not been fully amortized, the adjustment to the asset is recognized in income prospectively as a component of amortization. Historically, changes to non-landfill closure and post-closure estimates have not been material.

Remedial Liabilities

The balance of remedial liabilities at December 31, 2023 and 2022 was $111.2 million and $116.3 million, respectively. Remedial liabilities, including Superfund liabilities, include the costs of removal or containment of contaminated material, treatment of potentially contaminated groundwater and maintenance and monitoring costs necessary to comply with regulatory requirements. Most of the Company's remedial liabilities relate to the active and inactive hazardous waste treatment and disposal facilities which the Company acquired and Superfund sites owned by third-parties for which the Company, or the prior owners of certain of the Company's facilities for which the Company may have certain indemnification obligations, have been identified as potentially responsible parties ("PRPs") or potential PRPs. The Company's estimate of remedial liabilities involves an analysis of such factors as: (i) the nature and extent of environmental contamination (if any); (ii) the terms of applicable permits and agreements with regulatory authorities as to cleanup procedures and whether modifications to such permits and agreements will likely need to be negotiated; (iii) the cost of performing anticipated cleanup activities based upon current technology; and (iv) in the case of Superfund and other sites where other parties will also be responsible for a portion of the cleanup costs, the likely allocation of such costs and the ability of such other parties to pay their share. The measurement of remedial liabilities is reviewed at least quarterly and changes in estimates are recognized in the consolidated statements of operations when identified.

Where the Company concludes that it is probable that a liability has been incurred and an amount can be estimated, a liability is recognized. Remedial liabilities are inherently difficult to estimate. Estimating remedial liabilities requires that the existing environmental contamination be understood. There are risks that the actual quantities of contaminants differ from the results of the site investigation, and that contaminants exist that have not been previously identified. In addition, the value of the remedial liabilities is dependent on the remedial method selected. There is a risk that funds will be expended on a remedial solution that is not successful, which could result in the Company incurring the incremental costs of an alternative solution. Such estimates, which are subject to change, are subsequently revised if and when additional or new information becomes available.

Remedial liabilities are discounted when the timing of the payments is determinable and the amounts are estimable. In the case of remedial liabilities assumed in connection with acquisitions, acquired liabilities are recorded at fair value as of the dates of the acquisitions calculated by inflating costs in current dollars using an estimate of future inflation rates as of the respective acquisition dates until the expected time of payment, and then discounting the amount of the payments to their present value using a risk-free discount rate as of the acquisition dates. Discount rates used in the present value determination of the Company's remedial liabilities range from 1.37% to 4.90%.

Self Insurance Liabilities

Self-Insurance Liabilities

The Company self-insures a significant portion of expected losses related to workers' compensation, employee medical, comprehensive general liability and vehicle liability. Liabilities associated with these losses are recorded based on the Company's estimates of the ultimate cost to settle incurred claims. These recorded liabilities are estimated based on independent

actuarial estimates and judgments which consider the frequency and settlement amount of historical claims data and are discounted to present value using a risk-free interest rate.

Revenue recognition

Revenue Recognition

The Company recognizes revenue when control of the promised goods or services is transferred to the Company's customers, in an amount that reflects the consideration the Company expects to be entitled to in exchange for those goods or services. The majority of the Company’s revenues are for services, which are recognized based on time and materials incurred at contractually agreed-upon rates. Product revenues are recognized when the products are delivered and control transfers to the customer. The Company’s payment terms vary by the type of customers and the products or services offered. The periods between invoicing and when payments are due are not significant. Amounts billed to customers related to shipping and handling are classified as revenue, and the Company's shipping and handling costs are included in costs of revenues. In the course of operations, the Company collects sales tax and other excise taxes from its customers and recognizes a current liability, which is then relieved when the taxes are remitted to the appropriate government authorities. The Company excludes sales and other excise taxes that it collects from customers from its revenues.

Foreign Currency

The Company has international operations in Canada and administrative support services located in India. The functional currencies of foreign operations are the local currency and therefore assets and liabilities of those foreign operations are translated to U.S. Dollars at the exchange rate in effect at the balance sheet date and revenue and expenses at the average exchange rate for the period. Gains and losses from the translation of the consolidated financial statements of foreign subsidiaries into U.S. Dollars are included in stockholders' equity as a component of accumulated other comprehensive loss. Gains and losses from transactions not denominated in the functional currency of an entity are recognized in the consolidated statements of operations. Recorded balances that are denominated in a currency other than the functional currency are remeasured to the functional currency using the exchange rate at the balance sheet date and gains or losses are recorded in the consolidated statements of operations.

Defined Contribution Plan

The Company has defined contribution plans under which eligible employees may contribute up to the maximum amount as provided by law.

Advertising Expense

Advertising costs are expensed as incurred.

Stock-Based Compensation

Stock-based compensation cost is measured at the grant date based on the fair value of the award and is recognized as expense over the requisite service period, which generally represents the vesting period. In addition, the Company issues awards with performance targets established prior to or at the grant date. The expense for these awards is recognized over the requisite service period when management believes it is probable those performance targets will be achieved. The fair value of the Company's grants are based on the closing price of the Company's common stock on the respective dates of grant. Forfeitures are recognized as they occur. Stock-based compensation is recognized in selling, general and administrative expense.

Interest Rate Derivatives

The Company enters into interest rate derivative agreements as part of the overall strategy to hedge against fluctuations in variable interest rates. These interest rate derivatives swap a variable interest rate for a fixed interest rate and have been designated as cash flow hedges. The Company assesses the hedge’s effectiveness at the inception of the hedge and at regular intervals throughout the life of the derivative. To the extent that the interest rate swap is highly effective, changes in fair value are recorded in stockholders’ equity as a component of accumulated other comprehensive loss. Amounts are reclassified from accumulated other comprehensive loss into interest expense on the consolidated statement of operations in the same period or periods during which the hedged transactions affect earnings. Although it was determined that the interest rate swaps will be a highly effective hedge, any portion of the interest rate swaps subsequently determined to be ineffective will be recognized in

earnings. Further, if it becomes probable that a forecasted transaction designated as the hedged item will not occur, any gain or loss deferred is recognized in interest expense at that time.

The fair value of the interest rate swaps is calculated using discounted cash flow valuation methodologies based upon the one-month variable rate yield curves that are observable at commonly quoted intervals for the full term of the interest rate swaps and as such is considered a Level 2 measure according to the fair value hierarchy. The Company recognizes the fair value of the derivative instruments by counterparty as either a net asset, included in Other long-term assets, or net liability, included in Accrued expenses and other current liabilities, on the consolidated balance sheets.

Income Taxes

Current income tax expense approximates cash to be paid or refunded for taxes for the applicable period. Deferred tax expense or benefit is the result of changes between deferred tax assets and liabilities. Deferred tax assets and liabilities are determined based upon the temporary differences between the financial statement basis and tax basis of assets and liabilities as well as from net operating loss and tax credit carryforwards as measured by the enacted tax rates which will be in effect when these differences reverse. The effect of a change in tax rates on deferred tax assets and liabilities is generally recognized in income in the period that includes the enactment date. The Company evaluates the recoverability of future tax deductions and credits and a valuation allowance is established by tax jurisdiction when, based on an evaluation of both positive and negative objective verifiable evidence, it is more likely than not that some portion or all of deferred tax assets will not be realized.

The Company recognizes and measures a tax benefit from uncertain tax positions when it is more likely than not that the tax position will be sustained upon examination by the taxing authorities, based on the technical merits of the position. The Company recognizes a liability for unrecognized tax benefits resulting from uncertain tax positions taken or expected to be taken in a tax return. The Company adjusts these liabilities when its judgment changes as a result of the evaluation of new information not previously available. Due to the complexity of some of these uncertainties, the ultimate resolution may result in a payment that is materially different from the current estimate or future recognition of an unrecognized benefit. These differences will be reflected as increases or decreases to income tax expense in the period in which they are determined. Liabilities for unrecognized tax benefits are included within other long-term liabilities in the consolidated balance sheets.

The Company recognizes interest and penalties related to unrecognized tax benefits within the provision for income taxes line in the consolidated statements of operations. Accrued interest and penalties are included within the other long-term liabilities line in the consolidated balance sheets.

Earnings per Share ("EPS")

Basic EPS is calculated by dividing net income by the weighted average number of common shares outstanding during the period. Diluted EPS gives effect to all potentially dilutive common shares that were outstanding during the period.

Leases

The Company’s leases predominately relate to real estate, equipment, such as vehicles and industrial equipment utilized in operations, and rail cars utilized in connection with the Company’s transportation needs. Contracts are reviewed at inception to determine if the arrangement is a lease and, if so, whether it is an operating or finance lease. For all of its leases, the Company has elected not to separate lease and nonlease components, such as common area maintenance.

The Company generally enters into long-term real estate leases with two to ten-year terms and long-term non-real estate leases with three to eight-year terms. In the normal course of business, the Company also enters into short-term leases having terms of one year or less. These leases are generally equipment leases entered into for short periods of time (e.g. daily, weekly or monthly) to satisfy immediate and/or short-term operational needs of the business which can arise based upon the nature of particular services performed. The Company has elected not to recognize right-of-use ("ROU") assets and lease liabilities for these short-term leases. Operating and finance leases with terms exceeding one year are recognized as ROU assets and lease liabilities and measured at commencement date based on the present value of the future lease payments over the lease term.

Certain of the Company's real estate leases contain escalating future lease payments. Escalating lease payments that are based upon explicit amounts contained in the lease or an index (e.g., consumer price index) are included in the Company's determination of future lease payments to determine the ROU asset and lease liability recognized at the commencement date. Any differences in the future lease payments from initial recognition are not anticipated to be material and will be recorded as variable lease cost in the period incurred. The variable lease cost will also include the Company’s portion of property tax, utilities and common area maintenance. A significant portion of the Company’s real estate lease agreements include renewal periods at the Company’s option. The Company includes these renewal periods in the lease term only when renewal is

reasonably certain based upon facts and circumstances specific to the lease and known by the Company. Certain of the equipment and rail car leases transfer ownership upon the conclusion of the lease term and as such, are classified as finance leases. Leases containing purchase options are classified as finance leases only when it is reasonably certain that the Company will execute such options.

The Company uses its incremental borrowing rate on collateralized debt based on the information available at the lease commencement date in determining the present value of future lease payments as the implicit rate is typically not readily determinable.

For operating leases, lease cost is recognized on a straight-line basis over the lease term and is included in cost of revenues or selling, general and administrative expenses depending on the use of the asset. For finance leases, ROU assets are amortized on a straight-line basis and interest expense is recognized based on the incremental borrowing rate. Finance lease amortization is recognized in depreciation and amortization and finance lease related interest expense is recognized in interest expense, net of interest income.

Accounting Pronouncements Not Yet Adopted

In November 2023, the Financial Accounting Standards Board ("FASB") issued ASU No. 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures, which requires an enhanced disclosure of significant segment expenses on an annual and interim basis. This guidance will be effective for the annual periods beginning the year ended December 31, 2024, and for interim periods beginning January 1, 2025. Early adoption is permitted. Upon adoption, the guidance should be applied retrospectively to all prior periods presented in the financial statements. The Company is currently evaluating the impact of adopting this new pronouncement on its consolidated financial statements disclosures.

In December 2023, the FASB issued ASU No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures, which enhances income tax disclosures related to the tax rate reconciliation and income taxes paid. This guidance will be effective for the annual periods beginning the year ended December 31, 2025. Early adoption is permitted. Upon adoption, the guidance can be applied prospectively or retrospectively. The Company is currently evaluating the impact of adopting this new pronouncement on its consolidated financial statements disclosures.

STOCK-BASED COMPENSATION

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

STOCK-BASED COMPENSATION

Stock-Based Compensation

In 2020, the Company's shareholders approved the Clean Harbors, Inc. 2020 Stock Incentive Plan (the "2020 Plan"). The 2020 Plan provides for future awards of up to 2.5 million shares of the Company’s common stock (subject to certain anti-dilution adjustments) in the form of stock options, stock appreciation rights, restricted stock, restricted stock units and other stock-based awards. The 2020 Plan is administered by the Compensation and Human Capital Committee of the Company’s Board of Directors.

The Company grants restricted stock awards and performance stock awards. The restricted stock awards generally vest over three to five years subject to continued employment. The performance stock awards vest depending on the satisfaction of certain performance conditions typically based on revenue, Adjusted EBITDA margin, return on invested capital and a measure of workplace safety. In addition, performance stock awards include continued service conditions subsequent to the attainment of any performance conditions.

Total stock-based compensation cost recognized for the years ended December 31, 2023, 2022 and 2021 was $20.7 million, $26.8 million and $18.8 million, respectively. The total income tax benefit recognized in the consolidated statements of operations from stock-based compensation expense was $3.5 million, $5.0 million and $3.5 million for the years ended December 31, 2023, 2022 and 2021, respectively.

Restricted Stock Awards

The following table summarizes information about restricted stock awards for the year ended December 31, 2023:


Restricted Stock	Number of Shares	Weighted Average Grant-Date Fair Value
Balance at January 1, 2023	427,142	$	84.64
Granted	209,431	140.32
Vested	(170,060)	76.26
Forfeited	(51,890)	102.11
Balance at December 31, 2023	414,623	$	114.02

As of December 31, 2023, there was $34.5 million of total unrecognized compensation cost arising from restricted stock awards. This cost is expected to be recognized over a weighted average period of 2.9 years. The total fair value of restricted stock vested during 2023, 2022 and 2021 was $26.5 million, $16.4 million and $17.7 million, respectively.

Performance Stock Awards

Performance stock awards are subject to performance criteria established by the Compensation and Human Capital Committee of the Company's Board of Directors prior to or at the date of grant. The vesting of the performance stock awards is based on achieving targets currently based on revenue, Adjusted EBITDA Margin, Return on Invested Capital and Total Recordable Incident Rate. In addition, performance stock awards include continued service conditions.

The following table summarizes information about performance stock awards for the year ended December 31, 2023:


Performance Stock	Number of Shares	Weighted Average Grant-Date Fair Value
Balance at January 1, 2023	213,679	$	91.62
Granted	120,765	131.18
Vested	(111,924)	90.56
Forfeited	(41,236)	111.50
Balance at December 31, 2023	181,284	$	114.10

As of December 31, 2023, there was $3.3 million of total unrecognized compensation cost arising from unvested performance stock awards deemed probable of vesting. The total fair value of performance awards vested during 2023, 2022 and 2021 was $17.0 million, $11.1 million and $15.0 million, respectively.

FINANCING ARRANGEMENTS

12 Months Ended

Dec. 31, 2023

Debt Disclosure [Abstract]

FINANCING ARRANGEMENTS

Long-term Debt

The following table is a summary of the Company's long-term debt (in thousands):


	December 31, 2023		December 31, 2022
Current Portion of Long-Term Debt:
Secured senior term loans	$	10,000	$	10,000


Long-Term Debt:
Secured senior term loans due June 30, 2024 ("2024 Term Loans")	$	—	$	613,975
Secured senior term loans due October 8, 2028 ("2028 Term Loans")	970,000		980,000
Unsecured senior notes, at 4.875%, due July 15, 2027 ("2027 Notes")	545,000		545,000
Unsecured senior notes, at 5.125%, due July 15, 2029 ("2029 Notes")	300,000		300,000
Unsecured senior notes, at 6.375%, due February 1, 2031 ("2031 Notes")	500,000		—
Long-term debt, at par	2,315,000		2,438,975
Unamortized debt issuance costs and discount, net	(23,283)		(24,147)
Long-term debt, at carrying value	$	2,291,717	$	2,414,828

As of December 31, 2023 and 2022, the estimated fair value of the Company’s outstanding long-term debt, including the current portion, was $2.3 billion and $2.4 billion, respectively. The Company’s estimates of fair value of its long-term debt, including the current portion, are based on quoted market prices or other available market data which are considered Level 2 measures according to the fair value hierarchy. Level 2 utilizes quoted market prices in markets that are not active, broker or dealer quotation or alternative pricing sources with reasonable levels of price transparency for similar assets and liabilities.

The sections below will outline the key terms of the outstanding debt, including any changes occurring during the period:

Secured Senior Term Loans

As of December 31, 2023, the 2028 Term Loans had an aggregate principal amount outstanding of $980.0 million under the Fourth Amendment to the Company's existing Credit Agreement dated as of June 30, 2017 (the "Term Loan Agreement").

Interest on the 2028 Term Loans is paid monthly. On December 27, 2023, the Term Loan Agreement was amended to reduce the interest rate margin by 25 basis points. After giving effect to this amendment, the 2028 Term Loan bears interest, at the Company’s election, at either of the following rates: (a) the sum of the Term SOFR Rate (as defined) plus 0.11448% (the one-month Term SOFR adjustment as defined) plus a 1.75% margin, or (b) the sum of the Base Rate (as defined) plus 0.75% margin. The Term SOFR Rate is subject to a floor of 0.00% and the Base Rate is subject to a floor of 1.00%. The Term Loan Agreement provides for Term SOFR adjustments for other interest periods; however, the Company elects one-month Term SOFR for interest payments on that debt resulting in total interest on the 2028 Term Loans of one-month Term SOFR plus 1.86448%.

The 2028 Term Loans are prepayable at any time without premium or penalty, other than customary breakage costs. The 2028 Term Loan amendment in December 2023 reset the six month soft call period for repricing transactions.

On January 24, 2023, the Company repaid the $614.0 million outstanding of 2024 Term Loans using net proceeds from the issuance of $500.0 million of unsecured 2031 Senior Notes, as described below, borrowing $114.0 million under the Company's Revolving Credit Facility, also discussed below, and available cash. During the year, the Company recognized a loss on early extinguishment of debt of $2.9 million due to this transaction and the 2028 Term Loan amendment noted above.

The Company's obligations under the Term Loan Agreement are guaranteed by all of the Company's domestic restricted subsidiaries and secured by the liens on substantially all of the assets of the Company and the guarantors.

Unsecured Senior Notes

On July 2, 2019, the Company completed a private placement of $545.0 million aggregate principal amount of 2027 Notes and $300.0 million aggregate principal amount of 2029 Notes. The 2027 Notes will mature on July 15, 2027 and the 2029 Notes will mature on July 15, 2029. Interest payments on the 2027 and 2029 Notes are paid semiannually on January 15 and July 15.

On January 24, 2023, the Company completed a private placement of $500.0 million aggregate principal amount of 2031 Notes which will mature on February 1, 2031 (collectively referred to with the 2027 Notes and 2029 Notes as the "Notes"). As noted above, the proceeds from this were used toward the repayment of the 2024 Term Loans. The interest rate on the 2031 Notes is paid semiannually on February 1 and August 1 of each year and commenced on August 1, 2023.

The tables below depict the redemption prices of the Notes if redeemed during the twelve-month period commencing on the dates below, plus accrued and unpaid interest, if any, to but not including the redemption date.

2027 Notes


	Percentage

July 15, 2023	101.219	%
July 15, 2024 and thereafter	100.000	%

2029 Notes


	Percentage
July 15, 2024	102.563	%
July 15, 2025	101.281	%
July 15, 2026 and thereafter	100.000	%

2031 Notes


	Percentage
February 1, 2026	103.188	%
February 1, 2027	101.594	%
February 1, 2028 and thereafter	100.000	%

The Company may also redeem all or any portion of the 2029 Notes prior to July 15, 2024 or the 2031 Notes prior to February 1, 2026, at a redemption price equal to 100% of the principal amount redeemed plus a make whole premium as of the date of redemption including accrued and unpaid interest, if any, up to but not including the date of redemption.

Additionally, subject to certain limitations, prior to February 1, 2026, the Company may use net cash proceeds of one or more equity offerings to redeem up to 40% in aggregate principal of the 2031 Notes at a redemption price equal to 106.375% of the principal amount thereof plus accrued and unpaid interest thereon, if any, up to but not including the date of redemption.

The Notes and the related indentures contain various customary non-financial covenants and are guaranteed by substantially all of the Company’s current and future domestic subsidiaries. The Notes are effectively subordinated to the Company's Term Loans, revolving credit facility and finance lease obligations to the extent of the value of the assets securing such secured indebtedness. The Notes are also effectively subordinated to all indebtedness and other liabilities of the Company's subsidiaries that are not guarantors of the Notes.

Revolving Credit Facility.

On October 28, 2020, the Company and one of the Company's subsidiaries (the "Canadian Borrower") entered into an amended and restated credit agreement for the Company's revolving credit facility with Bank of America, N.A. (“BofA”), as agent for the lenders under the facility (the "Agent"). Under the amended and restated facility, the Company has the right to obtain revolving loans and letters of credit for a combined maximum of up to $350.0 million (with a sub-limit of $250.0 million for letters of credit) and the Canadian Borrower has the right to obtain revolving loans and letters of credit for a combined maximum of up to $50.0 million. Availability under the U.S. line is subject to a borrowing base primarily comprised of 85% of the eligible accounts receivable of the Company and its U.S. subsidiaries plus 100% of cash deposited in a controlled account with the Agent, and availability under the Canadian line is subject to a borrowing base primarily comprised of 85% of the eligible accounts receivable of the Company’s Canadian subsidiaries plus 100% of cash deposited in a controlled account with the Agent’s Canadian affiliate. Subject to certain conditions, the revolving credit facility will expire on October 28, 2025.

On April 28, 2023, the Company entered into an amendment to the credit agreement for the revolving credit facility. As amended, the terms of the agreement are substantially the same as prior to the amendment except for certain updates required to transition the agreement to include a defined LIBOR successor rate. Under the amended agreement, borrowings under the revolving credit facility will bear interest at a rate, at the Company’s option, of either (i) “Term SOFR” (as defined in the amended agreement) plus an applicable margin ranging from 1.50% to 1.75% per annum based primarily on the level of the Company’s average liquidity for the most recent 30 day period or (ii) BofA’s base rate plus an applicable margin ranging from

0.50% to 0.75% per annum based primarily on such average liquidity. The amended agreement also continues to provide (i) for an unused line fee payable to the lenders, calculated on the then unused portion of the lenders’ $400.0 million maximum commitments, ranging from 0.25% to 0.375% per annum of the unused commitment, and (ii) for outstanding letters of credit, a fee payable to the lenders equal to the then applicable margin for Term SOFR borrowings described above, and to the issuing banks a standard fronting fee and customary fees and charges in connection with all amendments, extensions, draws and other actions with respect to letters of credit.

Letters of credit issued under the revolving credit facility are utilized primarily as security for the Company's insurance program that includes casualty and financial assurance.

The Company’s obligations under the revolving credit facility (including revolving loans and reimbursement obligations for outstanding letters of credit) are guaranteed by substantially all of the Company’s U.S. subsidiaries and secured by a first lien on the Company’s and its U.S. subsidiaries’ accounts receivable. The Canadian Borrower’s obligations under the facility are guaranteed by substantially all of the Company’s Canadian subsidiaries and secured by a first lien on the accounts receivable of the Canadian subsidiaries.

The revolving credit facility had no outstanding loan balances at December 31, 2023 and had availability of $265.7 million and outstanding letters of credit of $134.3 million. The Company also had no outstanding loan balances as of December 31, 2022. As noted above, on January 24, 2023, the Company borrowed $114.0 million under the revolving credit facility. Proceeds from this borrowing were used toward the repayment of the 2024 Term Loans. This borrowing was repaid during the second quarter of 2023.

Cash Flow Hedges

The Company's strategy to hedge against fluctuations in variable interest rates involves entering into interest rate derivative agreements.

The Company effectively fixed the interest rate on $350.0 million principal of the previously outstanding 2024 Term Loans by entering into interest rate swap agreements in 2018 with a notional amount of $350.0 million ("2018 Swaps"). On January 24, 2023, concurrently with the repayment of the 2024 Term Loans, the Company received a settlement payment of $8.3 million from the 2018 Swap counterparties. As a result of the settlement, the Company also reclassified the amounts previously deferred in accumulated other comprehensive loss and recognized a settlement gain of $8.3 million in interest expense in 2023.

In 2022, the Company entered into interest rate swap agreements with a notional amount of $600.0 million ("2022 Swaps") to effectively fix the interest rate on $600.0 million principal of the outstanding 2028 Term Loans. Under the terms of the 2022 Swaps' agreements, the Company receives interest based upon the variable rates on the 2028 Term Loans and pays a fixed amount of interest. The fixed rate on these instruments was 0.931% through June 30, 2023 which, together with the original 2.00% interest rate margin for the 2028 Term Loans, resulted in an effective annual interest rate of approximately 2.931% through June 30, 2023.

The fixed rate on these instruments increased to 1.9645% on July 1, 2023 and the variable rate became linked to Term SOFR to mirror the variable interest payments for the 2028 Term Loans. The 2022 Swaps expire September 30, 2027. Including the 2.00% interest rate margin for borrowings on the 2028 Term Loans and the 0.11448% SOFR adjustment per the 2028 Term Loans, beginning July 1, 2023, the effective annual interest rate of this $600.0 million increased to 4.07898%. With the 2028 Term Loan amendment in December 2023 reducing the margin from 2.00% to 1.75%, discussed in the Term Loans section above, the effective annual interest rate on the swapped portion of 2028 Term Loans outstanding decreased to 3.82898%.

At the inception of these instruments, the Company designated both the 2018 Swaps and the 2022 Swaps (collectively referred to as the “Swaps”) as cash flow hedges. As of December 31, 2023 the Company recorded a derivative asset with a fair value of $35.5 million comprised only of the 2022 Swaps as the 2018 Swaps were settled in early 2023, as noted above. The balance in the derivative asset as of December 31, 2022 was $60.6 million, which included both of the Swaps.

No ineffectiveness has been identified on the Swaps and, therefore the change in fair value is recorded in stockholders' equity as a component of accumulated other comprehensive loss. Amounts are reclassified from accumulated other comprehensive loss into interest expense on the consolidated statement of operations in the same period or periods during which the hedged transactions affect earnings.

- Definition

The entire disclosure for information about short-term and long-term debt arrangements, which includes amounts of borrowings under each line of credit, note payable, commercial paper issue, bonds indenture, debenture issue, own-share lending arrangements and any other contractual agreement to repay funds, and about the underlying arrangements, rationale for a classification as long-term, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements, such as the effects of refinancing and noncompliance with debt covenants.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1B

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(c))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 470
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//470/tableOfContent

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1B

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1B

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1B

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1C
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1C

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1C
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1C

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1C
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1C

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1E
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1E

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1I
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1I

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1I
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1I

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1I
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1I

+ Details

ACCUMULATED OTHER COMPREHENSIVE LOSS - Reclassification out of AOCI (Details) - USD ($) $ in Thousands		12 Months Ended
	Jan. 24, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Interest rate swap
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Unrealized Gain on Fair Value of Interest Rate Hedges		$ 30,273	$ 683	$ (10,011)
2018 Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Unrealized Gain on Fair Value of Interest Rate Hedges	$ 8,300	$ 8,300

CLOSURE AND POST-CLOSURE LIABILITIES - Changes in Post-Closure Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Changes to post-closure liabilities
Balance at the beginning of the period	$ 118,801	$ 99,103
Liabilities assumed in acquisitions	574	55
Measurement period adjustments from prior period acquisitions	3,015	1,148
New asset retirement obligations	3,092	3,743
Accretion	9,477	8,752
Changes in estimates recorded to consolidated statement of operations	1,200	6,775
Changes in estimates recorded to consolidated balance sheet	(2,106)	4,961
Environmental expenditures	(15,684)	(5,151)
Currency translation and other	231	(585)
Balance at the end of the period	118,600	118,801
Landfill Retirement Liability
Changes to post-closure liabilities
Balance at the beginning of the period	62,251	53,425
Liabilities assumed in acquisitions	0	0
Measurement period adjustments from prior period acquisitions	0	0
New asset retirement obligations	3,092	3,743
Accretion	4,958	4,605
Changes in estimates recorded to consolidated statement of operations	1,147	1,063
Changes in estimates recorded to consolidated balance sheet	(3,706)	3,219
Environmental expenditures	(8,478)	(3,373)
Currency translation and other	179	(431)
Balance at the end of the period	59,443	62,251
Non-Landfill Retirement Liability
Changes to post-closure liabilities
Balance at the beginning of the period	56,550	45,678
Liabilities assumed in acquisitions	574	55
Measurement period adjustments from prior period acquisitions	3,015	1,148
New asset retirement obligations	0	0
Accretion	4,519	4,147
Changes in estimates recorded to consolidated statement of operations	53	5,712
Changes in estimates recorded to consolidated balance sheet	1,600	1,742
Environmental expenditures	(7,206)	(1,778)
Currency translation and other	52	(154)
Balance at the end of the period	$ 59,157	$ 56,550

INCOME TAXES - Valuation Allowance (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Foreign tax credits	$ 4,422	$ 7,666
Federal net operating losses	3,783	3,783
State net operating loss carryforwards	4,809	9,928
Foreign net operating loss carryforwards	17,464	15,488
Deferred tax assets of a Canadian subsidiary	4,489	5,339
Realized and unrealized capital losses	305	305
Total valuation allowance	$ 35,272	$ 42,509

CLOSURE AND POST-CLOSURE LIABILITIES - Anticipated Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Environmental Property Sale, Disposal or Abandonment Costs
2024	$ 15,698
2025	16,694
2026	14,750
2027	3,955
2028	13,628
Thereafter	252,110
Undiscounted closure and post-closure liabilities	316,835
Less: Discount at credit-adjusted risk-free rate	(114,325)
Less: Undiscounted estimated closure and post-closure liabilities relating to airspace not yet consumed	(83,910)
Present value of closure and post-closure liabilities	$ 118,600	$ 118,801	$ 99,103

EARNINGS PER SHARE

12 Months Ended

Dec. 31, 2023

Earnings Per Share [Abstract]

EARNINGS PER SHARE

The following are computations of basic and diluted earnings per share (in thousands, except for per share amounts):


	For the years ended December 31,
	2023		2022		2021
Numerator for basic and diluted earnings per share:
Net income	$	377,856	$	411,744	$	203,247
Denominator:
Weighted basic shares outstanding	54,071		54,223		54,514
Dilutive effect of equity-based compensation awards	311		264		247
Weighted dilutive shares outstanding	54,382		54,487		54,761

Basic earnings per share	$	6.99	$	7.59	$	3.73
Diluted earnings per share	$	6.95	$	7.56	$	3.71

The Company included all outstanding performance awards and restricted stock awards in the calculation of diluted earnings per share except for as shown in the table below:


	For the years ended December 31,
	2023	2022	2021
Antidilutive restricted stock awards	12,247	10,250	67,981
Performance stock awards for which performance criteria was not attained at reporting date	88,876	—	14,237

CONSOLIDATED STATEMENTS OF OPERATIONS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Statement [Abstract]
Interest income	$ 11,930	$ 4,607	$ 2,218

BUSINESS COMBINATIONS - HydroChemPSC (Details) - USD ($)		3 Months Ended
	Oct. 08, 2021	Mar. 31, 2022	Dec. 31, 2022
Secured senior term loans due October 8, 2028 ("2028 Term Loans") \| Secured debt
Business Acquisition [Line Items]
Aggregate principal amount			$ 600,000,000
HydroChemPSC
Business Acquisition [Line Items]
Purchase price to acquire business	$ 1,230,000,000
Purchase price adjustment		$ 5,000,000
HydroChemPSC \| Secured senior term loans due October 8, 2028 ("2028 Term Loans") \| Secured debt
Business Acquisition [Line Items]
Aggregate principal amount	$ 1,000,000,000

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 444,698	$ 492,603
Short-term marketable securities	106,101	62,033
Accounts receivable, net of allowances aggregating $42,209 and $45,253, respectively	983,111	964,603
Unbilled accounts receivable	107,859	107,010
Inventories and supplies	327,511	324,994
Prepaid expenses and other current assets	82,939	82,518
Total current assets	2,052,219	2,033,761
Property, plant and equipment, net	2,193,318	1,980,302
Other assets:
Operating lease right-of-use assets	187,060	166,181
Goodwill	1,287,736	1,246,878
Permits and other intangibles, net	602,797	620,782
Other long-term assets	59,739	81,803
Total other assets	2,137,332	2,115,644
Total assets	6,382,869	6,129,707
Current liabilities:
Current portion of long-term debt	10,000	10,000
Accounts payable	451,806	446,629
Deferred revenue	95,230	94,094
Accrued expenses and other current liabilities	397,157	396,716
Current portion of closure, post-closure and remedial liabilities	26,914	23,123
Current portion of operating lease liabilities	56,430	49,532
Total current liabilities	1,037,537	1,020,094
Other liabilities:
Closure and post-closure liabilities, less current portion of $13,556 and $13,205, respectively	105,044	105,596
Remedial liabilities, less current portion of $13,358 and $9,918, respectively	97,885	106,372
Long-term debt, less current portion	2,291,717	2,414,828
Operating lease liabilities, less current portion	131,743	119,259
Deferred tax liabilities	353,107	350,389
Other long-term liabilities	118,330	90,847
Total other liabilities	3,097,826	3,187,291
Commitments and contingent liabilities (See Note 18)
Stockholders' equity:
Common stock	539	541
Additional paid-in capital	459,728	504,240
Accumulated other comprehensive loss	(175,339)	(167,181)
Retained earnings	1,962,578	1,584,722
Total stockholders' equity	2,247,506	1,922,322
Total liabilities and stockholders' equity	$ 6,382,869	$ 6,129,707

ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES

12 Months Ended

Dec. 31, 2023

Payables and Accruals [Abstract]

ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES

Accrued expenses and other current liabilities consisted of the following (in thousands):


	December 31, 2023		December 31, 2022
Accrued insurance	$	107,658	$	92,909
Accrued interest	33,857		20,033
Accrued compensation and benefits	113,236		123,226
Accrued income, real estate, sales and other taxes	44,752		61,442

Accrued other	97,654		99,106
	$	397,157	$	396,716

As of December 31, 2023 and 2022, accrued insurance included employee medical insurance costs of $22.8 million and $16.4 million, respectively, and accruals for losses under workers' compensation, comprehensive general liability and vehicle liability self-insurance programs of $82.7 million and $74.9 million, respectively. The increase in accrued interest is due to the timing of interest payments associated with debt issued in 2023. Refer to Note 12, "Financing Arrangements" for more information. The decrease in accrued compensation and benefits is primarily due to a decrease in accrued incentive compensation.

LEASES - Lease Expense (Details) - USD ($) $ in Thousands	12 Months Ended
LEASES - Lease Expense (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Leases [Abstract]
Operating lease cost	$ 69,156	$ 59,041	$ 50,264
Finance lease cost:
Amortization of ROU assets	17,183	13,239	9,504
Interest on lease liabilities	4,051	3,011	2,544
Total finance lease cost	21,234	16,250	12,048
Short-term lease cost	167,379	158,736	102,913
Variable lease cost	4,887	4,353	3,546
Total lease cost	$ 262,656	$ 238,380	$ 168,771

INCOME TAXES - Provision for Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest [Abstract]
Domestic	$ 401,912	$ 406,206	$ 223,438
Foreign	101,367	131,792	46,277
Income from operations before provision for income taxes	503,279	537,998	269,715
Current:
Federal	64,164	52,237	42,480
State	25,496	26,980	18,126
Foreign	23,078	29,488	4,380
Current income taxes	112,738	108,705	64,986
Deferred
Federal	18,251	32,199	2,275
State	(9,049)	(2,432)	(4,777)
Foreign	3,483	(12,218)	3,984
Deferred income taxes	12,685	17,549	1,482
Provision for income taxes	$ 125,423	$ 126,254	$ 66,468
Effective tax rate	24.90%	23.50%	24.60%
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Tax expense at U.S. statutory rate	$ 105,689	$ 112,980	$ 56,640
State income taxes, net of federal benefit	18,067	19,831	12,101
Foreign rate differential	4,213	6,196	1,922
Valuation allowance	(7,699)	(18,769)	(9,139)
Uncertain tax position interest and penalties	(7)	(2,454)	263
Tax credits expired	1,653	2,768	2,530
Non-deductible compensation	2,898	2,754	2,326
Other	609	2,948	(175)
Provision for income taxes	$ 125,423	$ 126,254	$ 66,468

COMMITMENTS AND CONTINGENCIES - Superfund Proceedings (Details)	12 Months Ended
	Dec. 31, 2023 site
Superfund Proceedings
Superfund Proceedings
Number of sites subject to proceedings under federal or state superfund laws	131
Number of sites owned by the entity subject to proceedings under federal or state superfund laws	6
Number of sites owned by third parties subject to proceedings under federal or state superfund laws	125
Number of sites for which environmental remediation expense is settled	30
Third party sites requiring expenditure on remediation	13
Number of sites for which environmental remediation expense is not required	82
Number of sites which potential liability could exceed $1.0 million	3
Safety-Kleen Sustainability Solutions
Superfund Proceedings
Notices received from owners of third party sites seeking indemnification from the company	17

DISPOSITION OF BUSINESS	12 Months Ended
DISPOSITION OF BUSINESS	Dec. 31, 2023
Discontinued Operations and Disposal Groups [Abstract]
DISPOSITION OF BUSINESS	DISPOSITION OF BUSINESS On June 30, 2022, the Company completed the sale of a line of business as part of its continuous focus on divesting certain non-core operations. The divested line of business was previously included within the Environmental Sales & Service reporting unit of the Environmental Services segment. The Company determined that the disposition did not constitute a strategic shift and that the impact on the Company's overall operations and financial results was not material. Accordingly, the operations associated with the disposal were not reported in discontinued operations. The final purchase price for the line of business was $18.8 million, after settling working capital, of which $1.5 million was held in escrow, half of which was received in 2023, and the remaining amount received in early 2024. The gain on sale of $8.9 million, after accounting for the assets sold, liabilities transferred upon sale and transaction costs, was included in gain on sale of business in the Company's consolidated statement of operations for the year ended December 31, 2022.

Audit Information	12 Months Ended
Audit Information	Dec. 31, 2023
Audit Information [Abstract]
Auditor Firm ID	34
Auditor Name	Deloitte & Touche LLP
Auditor Location	Boston, Massachusetts

BUSINESS COMBINATIONS - Other 2021 Acquisition Activity (Details) - USD ($) $ in Thousands	Jan. 25, 2022	Jun. 29, 2021	Mar. 27, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Business Acquisition [Line Items]
Goodwill				$ 1,287,736	$ 1,246,878	$ 1,227,042
Vertex Energy, Inc.
Business Acquisition [Line Items]
Consideration to acquire assets		$ 140,000
Payment for acquisition termination fee	$ 3,000
March 27, 2021 Acquisition
Business Acquisition [Line Items]
Purchase price to acquire business			$ 22,800
Goodwill			$ 16,300

- Definition

+ References + Details

SEGMENT REPORTING (Tables)

12 Months Ended

Dec. 31, 2023

Segment Reporting [Abstract]

Schedule of Reconciliation of Third Party Revenues to Direct Revenues

The following tables reconcile third-party revenues to direct revenues for the years ended December 31, 2023, 2022 and 2021 (in thousands):


	For the year ended December 31, 2023
	Environmental Services		Safety-Kleen Sustainability Solutions		Corporate Items		Total
Third-party revenues	$	4,469,909	$	938,796	$	447	$	5,409,152
Intersegment revenues (expense), net	41,533		(41,533)		—		—
Direct revenues	$	4,511,442	$	897,263	$	447	$	5,409,152


	For the year ended December 31, 2022
	Environmental Services		Safety-Kleen Sustainability Solutions		Corporate Items		Total
Third-party revenues	$	4,144,973	$	1,021,125	$	507	$	5,166,605
Intersegment revenues (expense), net	26,733		(26,733)		—		—
Direct revenues	$	4,171,706	$	994,392	$	507	$	5,166,605


	For the year ended December 31, 2021
	Environmental Services		Safety-Kleen Sustainability Solutions		Corporate Items		Total
Third-party revenues	$	3,025,907	$	779,360	$	299	$	3,805,566
Intersegment revenues (expense), net	6,547		(6,547)		—		—
Direct revenues	$	3,032,454	$	772,813	$	299	$	3,805,566

Schedule of Reconciliation to Consolidated Statements of Income to Adjusted EBITDA

The following table presents Adjusted EBITDA information used by management by reported segment (in thousands):


	For the years ended December 31,
	2023		2022		2021
Adjusted EBITDA:
Environmental Services	$	1,101,608	$	953,053	$	659,718
Safety-Kleen Sustainability Solutions	172,873		306,327		227,354
Corporate Items	(261,911)		(237,252)		(210,466)
Total	1,012,570		1,022,128		676,606
Reconciliation to Consolidated Statements of Operations:

Accretion of environmental liabilities	13,667		12,943		11,745
Stock-based compensation	20,703		26,844		18,839
Depreciation and amortization	365,761		347,594		298,135

Income from operations	612,439		634,747		347,887
Other (income) expense, net	(2,315)		(2,472)		515
Loss on early extinguishment of debt	2,880		422		—
Gain on sale of business	—		(8,864)		—
Interest expense, net of interest income	108,595		107,663		77,657
Income from operations before provision for income taxes	$	503,279	$	537,998	$	269,715

Schedule of Segment Reporting Information, by Segment

The following table presents assets by reported segment and in the aggregate (in thousands):


	December 31, 2023		December 31, 2022
Property, plant and equipment, net
Environmental Services	$	1,651,026	$	1,467,641
Safety-Kleen Sustainability Solutions	411,415		398,660
Corporate Items	130,877		114,001
Total property, plant and equipment, net	$	2,193,318	$	1,980,302

Goodwill and Permits and other intangibles, net
Environmental Services
Goodwill	$	1,112,013	$	1,071,846
Permits and other intangibles, net	454,919		462,050
Total Environmental Services	1,566,932		1,533,896

Safety-Kleen Sustainability Solutions
Goodwill	$	175,723	$	175,032
Permits and other intangibles, net	147,878		158,732
Total Safety-Kleen Sustainability Solutions	323,601		333,764

Total	$	1,890,533	$	1,867,660

Schedule of Revenue from External Customers and Long-Lived Assets, by Geographical Areas


	December 31, 2023				December 31, 2022
	Total		% of Total		Total		% of Total
Property, plant and equipment, net
United States	$	1,972,221	89.9	%	$	1,765,291	89.1	%
Canada and other foreign	221,097		10.1		215,011		10.9
Total property, plant and equipment, net	$	2,193,318	100.0	%	$	1,980,302	100.0	%

Permits and other intangibles, net
United States	$	570,049	94.6	%	$	585,887	94.4	%
Canada	32,748		5.4		34,895		5.6
Total permits and other intangibles, net	$	602,797	100.0	%	$	620,782	100.0	%

Schedule of Long-lived Assets by Geographic Areas

The following table presents the total assets by geographical area (in thousands):


	December 31, 2023		December 31, 2022
United States	$	5,786,937	$	5,595,255
Canada and other foreign	595,932		534,452

Total	$	6,382,869	$	6,129,707

SIGNIFICANT ACCOUNTING POLICIES - Permits and Other Intangibles (Details)	Dec. 31, 2023
Minimum \| Permits
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	5 years
Minimum \| Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	2 years
Maximum \| Permits
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	30 years
Maximum \| Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	25 years

SEGMENT REPORTING - Additional Information (Details)	12 Months Ended
SEGMENT REPORTING - Additional Information (Details)	Dec. 31, 2023 segment
Segment Reporting [Abstract]
Number of operating segments	2

GOODWILL AND OTHER INTANGIBLE ASSETS - Finite-lived and Indefinite Lived Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Finite-lived intangible assets
Finite-lived intangible assets, cost	$ 896,809	$ 861,470
Accumulated Amortization	414,297	363,222
Finite-lived intangible assets, net	482,512	498,248
Total permits and other intangible assets, cost	1,017,094	984,004
Total permits and other intangible assets, net	602,797	620,782
Trademarks and trade names
Finite-lived intangible assets
Trademarks and trade names	120,285	122,534
Permits
Finite-lived intangible assets
Finite-lived intangible assets, cost	191,747	188,373
Accumulated Amortization	117,556	109,036
Finite-lived intangible assets, net	74,191	79,337
Customer and supplier relationships
Finite-lived intangible assets
Finite-lived intangible assets, cost	604,994	583,709
Accumulated Amortization	258,879	229,368
Finite-lived intangible assets, net	346,115	354,341
Other intangible assets
Finite-lived intangible assets
Finite-lived intangible assets, cost	100,068	89,388
Accumulated Amortization	37,862	24,818
Finite-lived intangible assets, net	$ 62,206	$ 64,570

ACCUMULATED OTHER COMPREHENSIVE LOSS (Tables)

12 Months Ended

Dec. 31, 2023

Equity [Abstract]

Schedule of Reclassification of Accumulated Other Comprehensive Income (Loss)

The changes in accumulated other comprehensive loss by component and related tax impacts for the years ended December 31, 2023, 2022 and 2021 were as follows (in thousands):


	Foreign Currency Translation Adjustments		Unrealized Gain (Loss) on Available-For-Sale Securities		Unrealized (Loss) Gain on Fair Value of Interest Rate Hedges		Unrealized Loss on Pension		Total
Balance at January 1, 2021	$	(176,234)	$	135	$	(33,629)	$	(1,749)	$	(211,477)
Other comprehensive (loss) income before reclassifications	(1,590)		(361)		6,235		1,411		5,695
Amounts reclassified out of accumulated other comprehensive loss	—		—		10,011		—		10,011
Tax benefit (provision)	—		76		—		(317)		(241)
Other comprehensive (loss) income	(1,590)		(285)		16,246		1,094		15,465
Balance at December 31, 2021	(177,824)		(150)		(17,383)		(655)		(196,012)
Other comprehensive (loss) income before reclassifications	(31,515)		(523)		78,711		415		47,088
Amounts reclassified out of accumulated other comprehensive loss	—		—		(683)		—		(683)
Tax benefit (provision)	—		110		(17,587)		(97)		(17,574)
Other comprehensive (loss) income	(31,515)		(413)		60,441		318		28,831
Balance at December 31, 2022	(209,339)		(563)		43,058		(337)		(167,181)
Other comprehensive income (loss) before reclassifications	9,000		708		5,094		(741)		14,061
Amounts reclassified out of accumulated other comprehensive loss	—		—		(30,273)		—		(30,273)
Tax (provision) benefit	—		(149)		8,012		191		8,054
Other comprehensive income (loss)	9,000		559		(17,167)		(550)		(8,158)
Balance at December 31, 2023	$	(200,339)	$	(4)	$	25,891	$	(887)	$	(175,339)

Schedule of Reclassification out of Accumulated Other Comprehensive Income


	For the years ended December 31,
Other Comprehensive Loss Component	2023		2022		2021		Location

Unrealized (Loss) Gain on Fair Value of Interest Rate Hedges (1)	$	30,273	$	683	$	(10,011)	Interest expense, net of interest income

___________________________________

SIGNIFICANT ACCOUNTING POLICIES

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

SIGNIFICANT ACCOUNTING POLICIES

The accompanying consolidated financial statements of the Company reflect the application of certain significant accounting policies as described below:

Principles of Consolidation

Use of Estimates

Cash, Cash Equivalents, Marketable Securities and Uncashed Checks


	December 31, 2023		December 31, 2022
Commercial paper	$	27,542	$	5,035
U.S. Treasury securities	—		28,973
Total cash equivalents	27,542		34,008


Municipal bonds	—		1,930
Commercial paper	56,172		24,075
Corporate notes and bonds	49,929		36,028
Total marketable securities	106,101		62,033
Total	$	133,643	$	96,041

Allowance for Doubtful Accounts and Revenue Allowance

The following table reflects the activity in the allowance for doubtful accounts and revenue allowance (in thousands):


	Allowance for Doubtful Accounts						Revenue Allowance
	2023		2022		2021		2023		2022		2021
Balance at January 1,	$	24,659	$	24,136	$	24,634	$	20,594	$	16,004	$	20,115
Additions charged to earnings	5,956		7,783		8,018		46,467		54,836		34,319
Deductions from reserves, net of recoveries	(8,047)		(7,260)		(8,516)		(47,420)		(50,246)		(38,430)
Balance at December 31,	$	22,568	$	24,659	$	24,136	$	19,641	$	20,594	$	16,004

Credit Concentration

Inventories and Supplies

Property, Plant and Equipment, net (excluding landfill assets and finance lease right-of-use assets)

The Company depreciates and amortizes the capitalized cost of these assets, using the straight-line method as follows:


Asset Classification						Estimated Useful Life
Buildings and building improvements
Buildings						20-42 years
Leasehold and building improvements						2-45 years
Vehicles						2-15 years
Equipment
Capitalized software and computer equipment						3-5 years
Containers and railcars						8-16 years
All other equipment						4-30 years
Furniture and fixtures						5-8 years

Business Combinations

Goodwill

Permits and Other Intangibles

Landfill Accounting


Facility Name	Location	Remaining Lives (Years)	Permitted Remaining Highly Probable Airspace (cubic yards) (in thousands)
Buttonwillow	California	20	4,999
Deer Trail	Colorado	21	1,420
Grassy Mountain	Utah	34	4,320
Kimball	Nebraska	23	547
Lambton	Ontario, Canada	46	4,350
Lone Mountain	Oklahoma	16	3,165
Ryley	Alberta, Canada	30	5,692
Sawyer	North Dakota	65	3,260
			27,753

In 2023, as expected, the Company's non-commercial landfill at Deer Park, Texas reached permitted capacity and is in the process of closing. This landfill has been excluded from the table above.

At December 31, 2023 and 2022, the Company had no cubic yards of permitted, but not highly probable, airspace.

The following table presents the remaining highly probable airspace from January 1, 2022 through December 31, 2023 (in thousands of cubic yards):


	2023	2022
Remaining capacity, beginning of year	28,270	23,784
Changes in highly probable airspace, net	464	5,579
Consumed	(981)	(1,093)
Remaining capacity, end of year	27,753	28,270

Non-Landfill Closure and Post-Closure Liabilities

Remedial Liabilities

Self-Insurance Liabilities

actuarial estimates and judgments which consider the frequency and settlement amount of historical claims data and are discounted to present value using a risk-free interest rate.

Revenue Recognition

Foreign Currency

Defined Contribution Plan

The Company has defined contribution plans under which eligible employees may contribute up to the maximum amount as provided by law. The Company matches a portion of these employee contributions and contributed $28.8 million, $25.3 million and $20.5 million in 2023, 2022 and 2021 respectively.

Advertising Expense

Advertising costs are expensed as incurred. Advertising expense was $10.9 million in 2023, $9.4 million in 2022 and $6.0 million in 2021.

Stock-Based Compensation

Interest Rate Derivatives

earnings. Further, if it becomes probable that a forecasted transaction designated as the hedged item will not occur, any gain or loss deferred is recognized in interest expense at that time.

Income Taxes

Earnings per Share ("EPS")

Leases

Accounting Pronouncements Not Yet Adopted

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Retained Earnings
Beginning balance (in shares) at Dec. 31, 2020		54,773,000
Beginning balance at Dec. 31, 2020	$ 1,341,551	$ 548	$ 582,749	$ (211,477)	$ 969,731
Increase (Decrease) in Stockholders' Equity
Net income	203,247				203,247
Other comprehensive income (loss)	15,465			15,465
Stock-based compensation	18,839		18,839
Issuance of common stock for restricted share vesting, net of employee tax withholding (in shares)		235,000
Issuance of common stock for restricted share vesting, net of employee tax withholdings	$ (10,805)	$ 2	(10,807)
Repurchases of common stock (in shares)	(600,000)	(589,000)
Repurchases of common stock	$ (54,410)	$ (6)	(54,404)
Ending balance (in shares) at Dec. 31, 2021		54,419,000
Ending balance at Dec. 31, 2021	1,513,887	$ 544	536,377	(196,012)	1,172,978
Increase (Decrease) in Stockholders' Equity
Net income	411,744				411,744
Other comprehensive income (loss)	28,831			28,831
Stock-based compensation	26,844		26,844
Issuance of common stock for restricted share vesting, net of employee tax withholding (in shares)		183,000
Issuance of common stock for restricted share vesting, net of employee tax withholdings	$ (8,801)	$ 2	(8,803)
Repurchases of common stock (in shares)	(500,000)	(537,000)
Repurchases of common stock	$ (50,183)	$ (5)	(50,178)
Ending balance (in shares) at Dec. 31, 2022	54,064,797	54,065,000
Ending balance at Dec. 31, 2022	$ 1,922,322	$ 541	504,240	(167,181)	1,584,722
Increase (Decrease) in Stockholders' Equity
Net income	377,856				377,856
Other comprehensive income (loss)	(8,158)			(8,158)
Stock-based compensation	20,703		20,703
Issuance of common stock for restricted share vesting, net of employee tax withholding (in shares)		193,000
Issuance of common stock for restricted share vesting, net of employee tax withholdings	$ (13,838)	$ 1	(13,839)
Repurchases of common stock (in shares)	(300,000)	(328,000)
Repurchases of common stock	$ (51,379)	$ (3)	(51,376)
Ending balance (in shares) at Dec. 31, 2023	53,929,703	53,930,000
Ending balance at Dec. 31, 2023	$ 2,247,506	$ 539	$ 459,728	$ (175,339)	$ 1,962,578

GOODWILL AND OTHER INTANGIBLE ASSETS - Changes to Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Goodwill [Roll Forward]
Goodwill, beginning of period	$ 1,246,878	$ 1,227,042
Increase from current period acquisitions	39,346	34,510
Measurement period adjustments from prior period acquisition	360	(6,762)
Decrease from disposition of business		(4,412)
Foreign currency translation	1,152	(3,500)
Goodwill, end of period	1,287,736	1,246,878
Environmental Services
Goodwill [Roll Forward]
Goodwill, beginning of period	1,071,846	1,085,534
Increase from current period acquisitions	39,346	0
Measurement period adjustments from prior period acquisition	0	(6,762)
Decrease from disposition of business		(4,412)
Foreign currency translation	821	(2,514)
Goodwill, end of period	1,112,013	1,071,846
Safety-Kleen Sustainability Solutions
Goodwill [Roll Forward]
Goodwill, beginning of period	175,032	141,508
Increase from current period acquisitions	0	34,510
Measurement period adjustments from prior period acquisition	360	0
Decrease from disposition of business		0
Foreign currency translation	331	(986)
Goodwill, end of period	$ 175,723	$ 175,032

- Definition

+ References + Details

REMEDIAL LIABILITIES

12 Months Ended

Dec. 31, 2023

Environmental Remediation Obligations [Abstract]

REMEDIAL LIABILITIES

The changes to remedial liabilities from January 1, 2022 through December 31, 2023 were as follows (in thousands):


	Remedial Liabilities for Landfill Sites		Remedial Liabilities for Inactive Sites		Remedial Liabilities (Including Superfund) for Non-Landfill Operations		Total
Balance at January 1, 2022	$	1,780	$	59,787	$	50,306	$	111,873
Liabilities assumed in acquisition of real estate	—		—		8,092		8,092
Measurement period adjustments from prior period acquisition	—		—		338		338
Accretion	86		2,498		1,607		4,191
Changes in estimates recorded to consolidated statement of operations	8		877		612		1,497
Environmental expenditures	(50)		(3,370)		(5,375)		(8,795)
Currency translation and other	—		(43)		(863)		(906)
Balance at December 31, 2022	1,824		59,749		54,717		116,290


Accretion	89		2,540		1,561		4,190
Changes in estimates recorded to consolidated statement of operations	19		1,713		1,896		3,628
Expenditures	(52)		(3,840)		(9,384)		(13,276)
Currency translation and other	—		115		296		411
Balance at December 31, 2023	$	1,880	$	60,277	$	49,086	$	111,243

In the year ended December 31, 2023, the Company increased its remedial liabilities for an inactive site and an active site by $1.1 million each due to changes in the estimates of the related liabilities to account for new information obtained regarding the ultimate remediation of these sites. In the year ended December 31, 2022, the $8.1 million liability assumed in acquisition relates to real estate that the Company acquired in 2022. In purchasing the property, the Company assumed a known associated remedial liability, which was contemplated in the purchase price.


Years ending December 31,
2024	$	13,898
2025	19,253
2026	9,006
2027	7,001
2028	7,767
Thereafter	71,737
Undiscounted remedial liabilities	128,662
Less: Discount at risk free rates	(17,419)
Total remedial liabilities	$	111,243


Type of Facility or Site	Remedial Liabilities		% of Total		Reasonably Possible Additional Liabilities (1)
Inactive facilities not used in active conduct of the Company's business, most of which were assumed through prior period acquisitions (24 facilities)	$	60,277	54.2	%	$	11,585
Facilities now used in active conduct of the Company's business (45 facilities)	43,299		38.9		9,080
Superfund sites (13 sites)	7,667		6.9		1,150
Total	$	111,243	100.0	%	$	21,815

___________________________________

(1)Amounts represent the high end of the range of management's best estimate of the reasonably possible additional liabilities.


Location	Type of Facility or Site	Remedial Liabilities(1)		% of Total		Reasonably Possible Additional Liabilities(2)
Baton Rouge, LA	Closed incinerator and landfill	$	27,883	25.1	%	$	4,868
Bridgeport, NJ	Closed incinerator	17,472		15.7		3,288
Mercier, Quebec	Idled incinerator	11,501		10.3		1,735
Linden, NJ	Operating solvent recycling center	9,614		8.6		2,187
Various	All other incinerators, landfills, wastewater treatment facilities and service centers (65 facilities)	37,106		33.4		8,587
Various	Superfund sites (each representing less than 5% of total liabilities) (13 sites)	7,667		6.9		1,150
Total		$	111,243	100.0	%	$	21,815

_________________________________

(1)$25.0 million of the $111.2 million remedial liabilities include estimates related to the legal and administrative proceedings discussed in Note 18, "Commitments and Contingencies."

(2)Amounts represent the high end of the range of management's best estimate of the reasonably possible additional liabilities.

Revisions to remediation reserve requirements may result in upward or downward adjustments to income from operations in any given period. The Company believes that its extensive experience in the environmental services business, as well as its involvement with a large number of sites, provides a reasonable basis for estimating its aggregate liability. It is possible, however, that future changes in available technology, regulatory or enforcement developments, the results of environmental studies or other factors could necessitate the recording of additional liabilities or the revision of currently recorded liabilities that could be material. Since the Company's satisfaction of the liabilities will occur over many years, the Company cannot reasonably predict the nature or extent of possible future events or the impact that those events, if any, might have on the current estimates of remedial liabilities.

- Definition

The entire disclosure for environmental loss contingencies, such as presence of hazardous waste, relevant information from reports issued by regulators, and estimated costs to achieve compliance with regulatory requirements. This element may be used for all of an entity's disclosures about environmental loss contingencies.

+ References

Reference 1: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Topic 410
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Subparagraph (d)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481931/410-30-50-10

Reference 2: http://www.xbrl.org/2003/role/recommendedDisclosureRef
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-Section 50
-Paragraph 10
-Subparagraph (d)(3)
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Reference 3: http://www.xbrl.org/2003/role/exampleRef
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Reference 4: http://www.xbrl.org/2003/role/exampleRef
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-URI https://asc.fasb.org//410-30/tableOfContent

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Reference 7: http://www.xbrl.org/2003/role/recommendedDisclosureRef
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Reference 8: http://www.xbrl.org/2003/role/recommendedDisclosureRef
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Reference 10: http://www.xbrl.org/2003/role/recommendedDisclosureRef
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Reference 14: http://www.xbrl.org/2003/role/exampleRef
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481899/410-30-55-17

+ Details

INCOME TAXES (Tables)

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

Schedule of Income before Income Tax, Domestic and Foreign

The domestic and foreign components of income before provision for income taxes were as follows (in thousands):


	For the years ended December 31,
	2023		2022		2021
Domestic	$	401,912	$	406,206	$	223,438
Foreign	101,367		131,792		46,277
Total	$	503,279	$	537,998	$	269,715

Schedule of Components of Provision for Income Taxes

The provision for income taxes consisted of the following (in thousands, except percentages):


	For the years ended December 31,
	2023		2022		2021
Current:
Federal	$	64,164	$	52,237	$	42,480
State	25,496		26,980		18,126
Foreign	23,078		29,488		4,380
	112,738		108,705		64,986
Deferred
Federal	18,251		32,199		2,275
State	(9,049)		(2,432)		(4,777)
Foreign	3,483		(12,218)		3,984
	12,685		17,549		1,482
Provision for income taxes	$	125,423	$	126,254	$	66,468

Effective tax rate	24.9%		23.5%		24.6%

Schedule of Effective Income Tax Rate Reconciliation

The Company's effective income tax rate varied from the amount computed using the statutory federal income tax rate of 21% as follows (in thousands):


	For the years ended December 31,
	2023		2022		2021
Tax expense at U.S. statutory rate	$	105,689	$	112,980	$	56,640
State income taxes, net of federal benefit	18,067		19,831		12,101
Foreign rate differential	4,213		6,196		1,922
Valuation allowance	(7,699)		(18,769)		(9,139)
Uncertain tax position interest and penalties	(7)		(2,454)		263
Tax credits expired	1,653		2,768		2,530
Non-deductible compensation	2,898		2,754		2,326
Other	609		2,948		(175)
Provision for income taxes	$	125,423	$	126,254	$	66,468

Schedule of Deferred Tax Assets and Liabilities

The components of the total net deferred tax assets and liabilities as of December 31, 2023 and 2022 were as follows (in thousands):


	2023		2022
Deferred tax assets:

Provision for doubtful accounts	$	10,882	$	11,544
Closure, post-closure and remedial liabilities	31,944		31,837
Operating lease liabilities	46,784		42,255
Accrued expenses	14,963		19,311
Accrued compensation and benefits	15,058		20,171
Net operating loss carryforwards(1)	39,042		41,585
Tax credit carryforwards(2)	6,531		8,903

Stock-based compensation	3,516		3,988
Other	4,843		7,487
Total deferred tax assets	173,563		187,081
Deferred tax liabilities:
Property, plant and equipment	(284,997)		(281,131)
Operating lease right-of-use assets	(46,584)		(41,939)
Interest rate swap asset	(9,576)		(17,587)
Permits and other intangible assets	(130,391)		(132,681)
Prepaid expenses	(12,372)		(12,088)
Total deferred tax liabilities	(483,920)		(485,426)
Total net deferred tax liability before valuation allowance	(310,357)		(298,345)
Less valuation allowance	(35,272)		(42,509)
Net deferred tax liabilities	$	(345,629)	$	(340,854)

___________________________________

(2)As of December 31, 2023, the foreign tax credit carryforwards of $4.4 million will begin to expire in 2024.

Schedule of Valuation Allowance

The components of the total valuation allowance as of December 31, 2023 and 2022 were as follows (in thousands):


	2023		2022
Allowance related to:
Foreign tax credits	$	4,422	$	7,666
Federal net operating losses	3,783		3,783
State net operating loss carryforwards	4,809		9,928
Foreign net operating loss carryforwards	17,464		15,488
Deferred tax assets of a Canadian subsidiary	4,489		5,339
Realized and unrealized capital losses	305		305
Total valuation allowance	$	35,272	$	42,509

STOCK-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Schedule of Restricted Stock

The following table summarizes information about restricted stock awards for the year ended December 31, 2023:


Restricted Stock	Number of Shares	Weighted Average Grant-Date Fair Value
Balance at January 1, 2023	427,142	$	84.64
Granted	209,431	140.32
Vested	(170,060)	76.26
Forfeited	(51,890)	102.11
Balance at December 31, 2023	414,623	$	114.02

Schedule of Performance Stock Awards

The following table summarizes information about performance stock awards for the year ended December 31, 2023:


Performance Stock	Number of Shares	Weighted Average Grant-Date Fair Value
Balance at January 1, 2023	213,679	$	91.62
Granted	120,765	131.18
Vested	(111,924)	90.56
Forfeited	(41,236)	111.50
Balance at December 31, 2023	181,284	$	114.10

DISPOSITION OF BUSINESS (Details) - USD ($) $ in Thousands	12 Months Ended
DISPOSITION OF BUSINESS (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jun. 30, 2022
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of business	$ 0	$ 8,864	$ 0
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| June 30, 2022 Disposition
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from disposition of business				$ 18,800
Consideration held in escrow				$ 1,500
Gain on sale of business		$ 8,900

EARNINGS PER SHARE - Reconciliation of Basic and Diluted Earnings Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Numerator for basic and diluted earnings per share:
Net income, basic	$ 377,856	$ 411,744	$ 203,247
Net income, diluted	$ 377,856	$ 411,744	$ 203,247
Denominator:
Weighted basic shares outstanding (in shares)	54,071	54,223	54,514
Dilutive effect of equity-based compensation awards (in shares)	311	264	247
Weighted dilutive shares outstanding (in shares)	54,382	54,487	54,761
Basic earnings per share (in dollars per share)	$ 6.99	$ 7.59	$ 3.73
Diluted earnings per share (in dollars per share)	$ 6.95	$ 7.56	$ 3.71

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
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-URI https://asc.fasb.org//1943274/2147483443/250-10-50-3

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org//1943274/2147482635/260-10-55-15

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities, and addition from assumption of issuance of common shares for dilutive potential common shares; of income (loss) available to common shareholders.

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CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Comprehensive Income [Abstract]
Net income	$ 377,856	$ 411,744	$ 203,247
Other comprehensive (loss) income, net of tax:
Unrealized gain (loss) on available-for-sale securities	559	(413)	(285)
Unrealized gain on fair value of interest rate hedges	5,163	61,124	6,235
Reclassification adjustment for interest rate hedge amounts realized in net income	(16,425)	(683)	10,011
Reclassification adjustment for settlement of interest rate hedges	(5,905)	0	0
Pension adjustments	(550)	318	1,094
Foreign currency translation adjustments	9,000	(31,515)	(1,590)
Other comprehensive (loss) income, net of tax	(8,158)	28,831	15,465
Comprehensive income	$ 369,698	$ 440,575	$ 218,712

STOCK-BASED COMPENSATION - Restricted Stock Awards (Details) - Restricted Stock - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Number of Shares
Unvested, beginning of period (in shares)	427,142
Granted (in shares)	209,431
Vested (in shares)	(170,060)
Forfeited (in shares)	(51,890)
Unvested, end of period (in shares)	414,623	427,142
Weighted Average Grant-Date Fair Value
Unvested, beginning of period (in dollars per share)	$ 84.64
Granted (in dollars per share)	140.32
Vested (in dollars per share)	76.26
Forfeited (in dollars per share)	102.11
Unvested, end of period (in dollars per share)	$ 114.02	$ 84.64
Unrecognized compensation cost	$ 34.5
Period for recognition (in years)	2 years 10 months 24 days
Fair value of share-based payment awards	$ 26.5	$ 16.4	$ 17.7

Pay vs Performance Disclosure - USD ($) $ in Thousands	12 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pay vs Performance Disclosure
Net income	$ 377,856	$ 411,744	$ 203,247

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(20))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483621/220-10-S99-2

Reference 39: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(22))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483589/942-220-S99-1

+ Details

SEGMENT REPORTING - Reconciliation to Consolidated Statements of Income to Adjusted EBITDA (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 1,012,570	$ 1,022,128	$ 676,606
Reconciliation to Consolidated Statements of Operations:
Accretion of environmental liabilities	13,667	12,943	11,745
Stock-based compensation	20,703	26,844	18,839
Depreciation and amortization	365,761	347,594	298,135
Income from operations	612,439	634,747	347,887
Other Nonoperating Income (Expense)	(2,315)	(2,472)	515
Loss on early extinguishment of debt	2,880	422	0
Gain on sale of business	0	(8,864)	0
Interest expense, net of interest income	108,595	107,663	77,657
Income from operations before provision for income taxes	503,279	537,998	269,715
Operating segments \| Environmental Services
Segment Reporting Information [Line Items]
Adjusted EBITDA	1,101,608	953,053	659,718
Operating segments \| Safety-Kleen Sustainability Solutions
Segment Reporting Information [Line Items]
Adjusted EBITDA	172,873	306,327	227,354
Corporate, non-segment
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ (261,911)	$ (237,252)	$ (210,466)

SIGNIFICANT ACCOUNTING POLICIES - Leases (Details)	Dec. 31, 2023
Real Estate \| Minimum
Property, Plant and Equipment [Line Items]
Lessee term	2 years
Real Estate \| Maximum
Property, Plant and Equipment [Line Items]
Lessee term	10 years
Non-Real Estate \| Minimum
Property, Plant and Equipment [Line Items]
Lessee term	3 years
Non-Real Estate \| Maximum
Property, Plant and Equipment [Line Items]
Lessee term	8 years

COMMITMENTS AND CONTINGENCIES - Legal and Administrative Proceedings (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) proceeding claim	Dec. 31, 2022 USD ($)
Legal and Administrative Proceedings
Loss contingency accrual	$ 32.4	$ 37.1
Product Liability Cases
Legal and Administrative Proceedings
Number of proceedings \| proceeding	70
Loss contingency, claims settled and dismissed, claims \| claim	24
Legal and Administrative Proceedings
Legal and Administrative Proceedings
Loss contingency accrual	$ 25.0	24.1
Federal, State, and Provincial Enforcement Actions
Legal and Administrative Proceedings
Loss contingency accrual	$ 7.4	$ 13.0
Number of proceedings \| proceeding	0

SIGNIFICANT ACCOUNTING POLICIES - Remedial Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Regulatory Liabilities [Line Items]
Accrual for environmental loss contingencies	$ 111,243	$ 116,290	$ 111,873
Minimum
Regulatory Liabilities [Line Items]
Remedial liabilities at acquisition, risk-free interest rate (as a percent)	1.37%
Maximum
Regulatory Liabilities [Line Items]
Remedial liabilities at acquisition, risk-free interest rate (as a percent)	4.90%

CLOSURE AND POST-CLOSURE LIABILITIES

12 Months Ended

Dec. 31, 2023

Asset Retirement Obligation Disclosure [Abstract]

CLOSURE AND POST-CLOSURE LIABILITIES

The changes to closure and post-closure liabilities (also referred to as "asset retirement obligations") from January 1, 2022 through December 31, 2023 were as follows (in thousands):


	Landfill Retirement Liability		Non-Landfill Retirement Liability		Total
Balance at January 1, 2022	$	53,425	$	45,678	$	99,103
Liabilities assumed in acquisitions	—		55		55
Measurement period adjustments from prior period acquisitions	—		1,148		1,148
New asset retirement obligations	3,743		—		3,743
Accretion	4,605		4,147		8,752
Changes in estimates recorded to consolidated statement of operations	1,063		5,712		6,775
Changes in estimates recorded to consolidated balance sheet	3,219		1,742		4,961
Environmental expenditures	(3,373)		(1,778)		(5,151)
Currency translation and other	(431)		(154)		(585)
Balance at December 31, 2022	62,251		56,550		118,801
Liabilities assumed in acquisitions	—		574		574
Measurement period adjustments from prior period acquisitions	—		3,015		3,015
New asset retirement obligations	3,092		—		3,092
Accretion	4,958		4,519		9,477
Changes in estimates recorded to consolidated statement of operations	1,147		53		1,200
Changes in estimates recorded to consolidated balance sheet	(3,706)		1,600		(2,106)
Environmental expenditures	(8,478)		(7,206)		(15,684)
Currency translation and other	179		52		231
Balance at December 31, 2023	$	59,443	$	59,157	$	118,600

During the first quarter of 2023, the Company's non-commercial landfill at the Deer Park, Texas incineration facility reached its permitted capacity, as expected. The Company has commenced closure activities; however, there have been no changes to the liabilities related to this location. In the year ended December 31, 2023, there were no significant benefits or charges resulting from changes in estimates for closure and post-closure liabilities.


Years ending December 31,
2024	$	15,698
2025	16,694
2026	14,750
2027	3,955
2028	13,628
Thereafter	252,110
Undiscounted closure and post-closure liabilities	316,835
Less: Discount at credit-adjusted risk-free rate	(114,325)
Less: Undiscounted estimated closure and post-closure liabilities relating to airspace not yet consumed	(83,910)
Present value of closure and post-closure liabilities	$	118,600

SIGNIFICANT ACCOUNTING POLICIES - Landfill Accounting and Non-Landfill Closure and Post-Closure Liabilities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) cubic_yard landfill_site	Dec. 31, 2022 USD ($) cubic_yard	Dec. 31, 2021 USD ($) cubic_yard
Property, Plant and Equipment [Line Items]
Landfill sites \| landfill_site	8
Permitted, but not highly probable airspace (cubic yards)	0	0
Remaining Airspace Capacity [Roll Forward]
Remaining capacity, beginning of period (cubic yards)	28,270,000	23,784,000
Changes in highly probable airspace, net (cubic yards)	464,000	5,579,000
Consumed (cubic yards)	(981,000)	(1,093,000)
Remaining capacity, end of period (cubic yards)	27,753,000	28,270,000	23,784,000
Landfill final closure and post-closure liabilities \| $	$ 59.4	$ 62.3
Regulatory post-closure period for landfill (in years)	30 years
Asset retirement obligations, risk free rate (as a percent)	6.51%	5.37%
Non-landfill closure and post-closure liabilities \| $	$ 59.2	$ 56.6
Minimum
Remaining Airspace Capacity [Roll Forward]
Non-landfill closure and post-closure liabilities, period (in years)	10 years
Maximum
Remaining Airspace Capacity [Roll Forward]
Non-landfill closure and post-closure liabilities, period (in years)	30 years
Buttonwillow \| California
Property, Plant and Equipment [Line Items]
Remaining Lives (Years)	20 years
Deer Trail \| Colorado
Property, Plant and Equipment [Line Items]
Remaining Lives (Years)	21 years
Grassy Mountain \| Utah
Property, Plant and Equipment [Line Items]
Remaining Lives (Years)	34 years
Kimball \| Nebraska
Property, Plant and Equipment [Line Items]
Remaining Lives (Years)	23 years
Lambton \| Ontario, Canada
Property, Plant and Equipment [Line Items]
Remaining Lives (Years)	46 years
Lone Mountain \| Oklahoma
Property, Plant and Equipment [Line Items]
Remaining Lives (Years)	16 years
Ryley \| Alberta, Canada
Property, Plant and Equipment [Line Items]
Remaining Lives (Years)	30 years
Sawyer \| North Dakota
Property, Plant and Equipment [Line Items]
Remaining Lives (Years)	65 years
Landfill assets
Property, Plant and Equipment [Line Items]
Amortization \| $	$ 18.6	$ 16.2	$ 13.7
Permitted
Property, Plant and Equipment [Line Items]
Remaining highly probable airspace	27,753,000
Permitted \| Buttonwillow \| California
Property, Plant and Equipment [Line Items]
Remaining highly probable airspace	4,999,000
Permitted \| Deer Trail \| Colorado
Property, Plant and Equipment [Line Items]
Remaining highly probable airspace	1,420,000
Permitted \| Grassy Mountain \| Utah
Property, Plant and Equipment [Line Items]
Remaining highly probable airspace	4,320,000
Permitted \| Kimball \| Nebraska
Property, Plant and Equipment [Line Items]
Remaining highly probable airspace	547,000
Permitted \| Lambton \| Ontario, Canada
Property, Plant and Equipment [Line Items]
Remaining highly probable airspace	4,350,000
Permitted \| Lone Mountain \| Oklahoma
Property, Plant and Equipment [Line Items]
Remaining highly probable airspace	3,165,000
Permitted \| Ryley \| Alberta, Canada
Property, Plant and Equipment [Line Items]
Remaining highly probable airspace	5,692,000
Permitted \| Sawyer \| North Dakota
Property, Plant and Equipment [Line Items]
Remaining highly probable airspace	3,260,000

PROPERTY, PLANT, AND EQUIPMENT (Tables)

12 Months Ended

Dec. 31, 2023

Property, Plant and Equipment [Abstract]

Schedule of Property, Plant and Equipment

The Company depreciates and amortizes the capitalized cost of these assets, using the straight-line method as follows:


Asset Classification						Estimated Useful Life
Buildings and building improvements
Buildings						20-42 years
Leasehold and building improvements						2-45 years
Vehicles						2-15 years
Equipment
Capitalized software and computer equipment						3-5 years
Containers and railcars						8-16 years
All other equipment						4-30 years
Furniture and fixtures						5-8 years

Property, plant and equipment consisted of the following (in thousands):


	December 31, 2023		December 31, 2022
Land	$	174,891	$	172,579
Asset retirement costs (non-landfill)	27,167		22,001
Landfill assets	253,180		232,872
Buildings and improvements (1)	630,525		591,397

Vehicles (2)	1,276,567		1,112,188
Equipment (3)	2,388,370		2,195,064

Construction in progress	213,601		140,328
	4,964,301		4,466,429
Less - accumulated depreciation and amortization	2,770,983		2,486,127
Total property, plant and equipment, net	$	2,193,318	$	1,980,302

___________________________________

(1) Balances inclusive of gross ROU assets classified as finance leases of $8.0 million.

(2) Balances inclusive of gross ROU assets classified as finance leases of $151.7 million and $106.7 million, respectively.

(3) Balances inclusive of gross ROU assets classified as finance leases of $9.2 million.

REVENUES - Narrative (Details) $ in Millions	12 Months Ended
REVENUES - Narrative (Details) $ in Millions	Dec. 31, 2023 USD ($) source	Dec. 31, 2022 USD ($)
Disaggregation of Revenue [Line Items]
Deferred contract cost, recognition period	3 months
Deferred contracts costs \| $	$ 29.9	$ 29.7
Environmental Services
Disaggregation of Revenue [Line Items]
Number of revenue sources	4
Safety-Kleen Sustainability Solutions
Disaggregation of Revenue [Line Items]
Number of revenue sources	2

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References + Details

BUSINESS COMBINATIONS - Assets Acquired and Liabilities Assumed (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Oct. 08, 2021
Business Acquisition [Line Items]
Goodwill		$ 1,287,736,000	$ 1,246,878,000	$ 1,227,042,000
Measurement Period Adjustments, Goodwill		360,000	(6,762,000)
Thompson Industrial
Business Acquisition [Line Items]
Accounts receivable, including unbilled receivables		25,233,000
Inventories and supplies		228,000
Prepaid expenses and other current assets		1,302,000
Property, plant and equipment		26,719,000
Other intangibles		28,900,000
Operating lease right-of-use assets		4,716,000
Other long-term assets		72,000
Current liabilities		(10,385,000)
Current portion of operating lease liabilities		(1,653,000)
Operating lease liabilities, less current portion		(3,063,000)
Other long-term liabilities		(560,000)
Total identifiable net assets		71,509,000
Goodwill		39,346,000
Total purchase price		110,855,000
June 17, 2022 Acquisition
Business Acquisition [Line Items]
Accounts receivable, including unbilled receivables		1,089,000	1,111,000
Inventories and supplies		5,745,000	5,816,000
Prepaid expenses and other current assets		144,000	144,000
Property, plant and equipment		22,231,000	19,605,000
Other intangibles		23,500,000	23,500,000
Operating lease right-of-use assets		585,000	585,000
Other long-term assets		13,000	13,000
Current liabilities		(3,375,000)	(3,271,000)
Current portion of operating lease liabilities		(186,000)	(186,000)
Operating lease liabilities, less current portion		(399,000)	(399,000)
Other long-term liabilities		(2,681,000)	(55,000)
Total identifiable net assets		46,666,000	46,863,000
Goodwill		32,212,000	32,015,000
Total purchase price		78,878,000	$ 78,878,000
Measurement Period Adjustments, Accounts receivable, including unbilled receivables		(22,000)
Measurement Period Adjustments, Inventories and supplies		(71,000)
Measurement Period Adjustments, Prepaid expenses and other current assets		0
Measurement Period Adjustments, Property, plant and equipment		2,626,000
Measurement Period Adjustments, Permits and other intangibles		0
Measurement Period Adjustments, Operating lease right-of-use assets		0
Measurement Period Adjustments, Other non-current assets		0
Measurement Period Adjustments, Current liabilities		(104,000)
Measurement Period Adjustments, Current portion of operating lease liabilities		0
Measurement Period Adjustments, Operating lease liabilities, less current portion		0
Measurement Period Adjustments, Other long-term liabilities		(2,626,000)
Measurement Period Adjustments, Total identifiable net assets		(197,000)
Measurement Period Adjustments, Goodwill		197,000
Measurement Period Adjustments, Total purchase price		0
HydroChemPSC
Business Acquisition [Line Items]
Accounts receivable, including unbilled receivables		131,516,000
Inventories and supplies		3,162,000
Prepaid expenses and other current assets		16,291,000
Property, plant and equipment		314,397,000
Other intangibles		289,000,000
Operating lease right-of-use assets		34,415,000
Other long-term assets		962,000
Current liabilities		(118,854,000)
Current portion of operating lease liabilities		(11,277,000)
Operating lease liabilities, less current portion		(26,344,000)
Deferred tax liabilities		(80,386,000)
Other long-term liabilities		(4,170,000)
Total identifiable net assets		548,712,000
Goodwill		676,701,000
Total purchase price		1,225,413,000
Measurement Period Adjustments, Total purchase price	$ (5,000,000)
Goodwill, deductible for tax purposes		$ 0
HydroChemPSC \| Environmental sales & service reporting unit
Business Acquisition [Line Items]
Goodwill					$ 676,700,000

BUSINESS COMBINATIONS (Tables)

12 Months Ended

Dec. 31, 2023

Business Combination and Asset Acquisition [Abstract]

Schedule of Recognized Identified Assets Acquired and Liabilities Assumed

The following table summarizes the preliminary determination and recognition of assets acquired and liabilities assumed (in thousands):


	At Acquisition Date As Reported December 31, 2023
Accounts receivable	$	25,233
Inventories and supplies	228
Prepaid expenses and other current assets	1,302
Property, plant and equipment	26,719
Permits and other intangibles	28,900
Operating lease right-of-use assets	4,716
Other long-term assets	72
Current liabilities	(10,385)
Current portion of operating lease liabilities	(1,653)
Operating lease liabilities, less current portion	(3,063)

Other long-term liabilities	(560)
Total identifiable net assets	71,509
Goodwill	39,346
Total purchase price	$	110,855

The following table summarizes the final determination and recognition of assets acquired and liabilities assumed (in thousands):


	Preliminary Allocation As Reported December 31, 2022		Measurement Period Adjustments		Final Allocation As Reported December 31, 2023
Accounts receivable	$	1,111	$	(22)	$	1,089
Inventories and supplies	5,816		(71)		5,745
Prepaid expenses and other current assets	144		—		144
Property, plant and equipment	19,605		2,626		22,231
Permits and other intangibles	23,500		—		23,500
Operating lease right-of-use assets	585		—		585
Other long-term assets	13		—		13
Current liabilities	(3,271)		(104)		(3,375)
Current portion of operating lease liabilities	(186)		—		(186)
Operating lease liabilities, less current portion	(399)		—		(399)

Other long-term liabilities	(55)		(2,626)		(2,681)
Total identifiable net assets	46,863		(197)		46,666
Goodwill	32,015		197		32,212
Total purchase price	$	78,878	$	—	$	78,878

The following table summarizes the final determination and recognition of assets acquired and liabilities assumed (in thousands):


	Final Allocation
Accounts receivable, including unbilled receivables	$	131,516
Inventories and supplies	3,162
Prepaid expenses and other current assets	16,291
Property, plant and equipment	314,397
Other intangibles	289,000
Operating lease right-of-use assets	34,415
Other long-term assets	962
Current liabilities	(118,854)
Current portion of operating lease liabilities	(11,277)
Operating lease liabilities, less current portion	(26,344)
Deferred tax liabilities	(80,386)
Other long-term liabilities	(4,170)
Total identifiable net assets	548,712
Goodwill (i)	676,701
Total purchase price	$	1,225,413

_____________

Schedule of Unaudited Pro Forma Financial Information


	2021
Pro forma combined revenues	$	4,380,724
Pro forma combined net income	229,807

ACCUMULATED OTHER COMPREHENSIVE LOSS - Components of AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,922,322	$ 1,513,887	$ 1,341,551
Other comprehensive income (loss) before reclassifications	14,061	47,088	5,695
Amounts reclassified out of accumulated other comprehensive loss	(30,273)	(683)	10,011
Tax (provision) benefit	8,054	(17,574)	(241)
Other comprehensive (loss) income, net of tax	(8,158)	28,831	15,465
Ending balance	2,247,506	1,922,322	1,513,887
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(209,339)	(177,824)	(176,234)
Other comprehensive income (loss) before reclassifications	9,000	(31,515)	(1,590)
Amounts reclassified out of accumulated other comprehensive loss	0	0	0
Tax (provision) benefit	0	0	0
Other comprehensive (loss) income, net of tax	9,000	(31,515)	(1,590)
Ending balance	(200,339)	(209,339)	(177,824)
Unrealized Gain (Loss) on Available-For-Sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(563)	(150)	135
Other comprehensive income (loss) before reclassifications	708	(523)	(361)
Amounts reclassified out of accumulated other comprehensive loss	0	0	0
Tax (provision) benefit	(149)	110	76
Other comprehensive (loss) income, net of tax	559	(413)	(285)
Ending balance	(4)	(563)	(150)
Unrealized (Loss) Gain on Fair Value of Interest Rate Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	43,058	(17,383)	(33,629)
Other comprehensive income (loss) before reclassifications	5,094	78,711	6,235
Amounts reclassified out of accumulated other comprehensive loss	(30,273)	(683)	10,011
Tax (provision) benefit	8,012	(17,587)	0
Other comprehensive (loss) income, net of tax	(17,167)	60,441	16,246
Ending balance	25,891	43,058	(17,383)
Unrealized Loss on Pension
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(337)	(655)	(1,749)
Other comprehensive income (loss) before reclassifications	(741)	415	1,411
Amounts reclassified out of accumulated other comprehensive loss	0	0	0
Tax (provision) benefit	191	(97)	(317)
Other comprehensive (loss) income, net of tax	(550)	318	1,094
Ending balance	(887)	(337)	(655)
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(167,181)	(196,012)	(211,477)
Other comprehensive (loss) income, net of tax	(8,158)	28,831	15,465
Ending balance	$ (175,339)	$ (167,181)	$ (196,012)

BUSINESS COMBINATIONS - Unaudited Pro Forma Financial Information (Details) - HydroChemPSC $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($)
Business Acquisition [Line Items]
Pro forma combined revenues	$ 4,380,724
Pro forma combined net income	$ 229,807

PROPERTY, PLANT AND EQUIPMENT - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Abstract]
Depreciation expense, inclusive of landfill and finance lease amortization	$ 315.5	$ 297.4	$ 263.4
Interest costs capitalized	$ 6.6	$ 3.4	$ 0.0

LEASES - Other Information related to Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Weighted Average Remaining Lease Term (years)
Operating lease (in years)	3 years 9 months 18 days	4 years 2 months 12 days
Finance lease (in years)	7 years 1 month 6 days	7 years 6 months
Weighted Average Discount Rate
Operating lease (as a percentage)	4.68%	4.08%
Finance lease (as a percentage)	3.96%	3.53%
Cash paid for amounts related to lease liabilities:
Operating cash flows from operating leases	$ 71,185	$ 61,014	$ 50,963
Operating cash flows from finance leases	4,051	3,011	2,544
Financing cash flows from finance leases	15,937	12,821	8,458
ROU assets obtained in exchange for operating lease liabilities	82,392	58,230	55,556
ROU assets obtained in exchange for finance lease liabilities	$ 47,238	$ 29,205	$ 30,476

SIGNIFICANT ACCOUNTING POLICIES - Allowances for Doubtful Accounts And Revenue Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Allowance for Doubtful Accounts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at January 1,	$ 24,659	$ 24,136	$ 24,634
Additions charged to earnings	5,956	7,783	8,018
Deductions from reserves, net of recoveries	(8,047)	(7,260)	(8,516)
Balance at December 31,	22,568	24,659	24,136
Revenue Allowance
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at January 1,	20,594	16,004	20,115
Additions charged to earnings	46,467	54,836	34,319
Deductions from reserves, net of recoveries	(47,420)	(50,246)	(38,430)
Balance at December 31,	$ 19,641	$ 20,594	$ 16,004

SIGNIFICANT ACCOUNTING POLICIES (Tables)

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Schedule of Marketable Securities

As of December 31, 2023 and 2022, the Company had total marketable securities as follows (in thousands):


	December 31, 2023		December 31, 2022
Commercial paper	$	27,542	$	5,035
U.S. Treasury securities	—		28,973
Total cash equivalents	27,542		34,008


Municipal bonds	—		1,930
Commercial paper	56,172		24,075
Corporate notes and bonds	49,929		36,028
Total marketable securities	106,101		62,033
Total	$	133,643	$	96,041

Schedule of Activity in Allowance for Doubtful Accounts

The following table reflects the activity in the allowance for doubtful accounts and revenue allowance (in thousands):


	Allowance for Doubtful Accounts						Revenue Allowance
	2023		2022		2021		2023		2022		2021
Balance at January 1,	$	24,659	$	24,136	$	24,634	$	20,594	$	16,004	$	20,115
Additions charged to earnings	5,956		7,783		8,018		46,467		54,836		34,319
Deductions from reserves, net of recoveries	(8,047)		(7,260)		(8,516)		(47,420)		(50,246)		(38,430)
Balance at December 31,	$	22,568	$	24,659	$	24,136	$	19,641	$	20,594	$	16,004

Schedule of Activity in Revenue Allowance

The following table reflects the activity in the allowance for doubtful accounts and revenue allowance (in thousands):


	Allowance for Doubtful Accounts						Revenue Allowance
	2023		2022		2021		2023		2022		2021
Balance at January 1,	$	24,659	$	24,136	$	24,634	$	20,594	$	16,004	$	20,115
Additions charged to earnings	5,956		7,783		8,018		46,467		54,836		34,319
Deductions from reserves, net of recoveries	(8,047)		(7,260)		(8,516)		(47,420)		(50,246)		(38,430)
Balance at December 31,	$	22,568	$	24,659	$	24,136	$	19,641	$	20,594	$	16,004

Schedule of Asset Classification and Estimated Useful Life

The Company depreciates and amortizes the capitalized cost of these assets, using the straight-line method as follows:


Asset Classification						Estimated Useful Life
Buildings and building improvements
Buildings						20-42 years
Leasehold and building improvements						2-45 years
Vehicles						2-15 years
Equipment
Capitalized software and computer equipment						3-5 years
Containers and railcars						8-16 years
All other equipment						4-30 years
Furniture and fixtures						5-8 years

Property, plant and equipment consisted of the following (in thousands):


	December 31, 2023		December 31, 2022
Land	$	174,891	$	172,579
Asset retirement costs (non-landfill)	27,167		22,001
Landfill assets	253,180		232,872
Buildings and improvements (1)	630,525		591,397

Vehicles (2)	1,276,567		1,112,188
Equipment (3)	2,388,370		2,195,064

Construction in progress	213,601		140,328
	4,964,301		4,466,429
Less - accumulated depreciation and amortization	2,770,983		2,486,127
Total property, plant and equipment, net	$	2,193,318	$	1,980,302

___________________________________

(1) Balances inclusive of gross ROU assets classified as finance leases of $8.0 million.

(2) Balances inclusive of gross ROU assets classified as finance leases of $151.7 million and $106.7 million, respectively.

(3) Balances inclusive of gross ROU assets classified as finance leases of $9.2 million.

Schedule of Active Landfill Sites


Facility Name	Location	Remaining Lives (Years)	Permitted Remaining Highly Probable Airspace (cubic yards) (in thousands)
Buttonwillow	California	20	4,999
Deer Trail	Colorado	21	1,420
Grassy Mountain	Utah	34	4,320
Kimball	Nebraska	23	547
Lambton	Ontario, Canada	46	4,350
Lone Mountain	Oklahoma	16	3,165
Ryley	Alberta, Canada	30	5,692
Sawyer	North Dakota	65	3,260
			27,753

Schedule of Remaining Highly Probable Airspace

The following table presents the remaining highly probable airspace from January 1, 2022 through December 31, 2023 (in thousands of cubic yards):


	2023	2022
Remaining capacity, beginning of year	28,270	23,784
Changes in highly probable airspace, net	464	5,579
Consumed	(981)	(1,093)
Remaining capacity, end of year	27,753	28,270

EARNINGS PER SHARE (Tables)

12 Months Ended

Dec. 31, 2023

Earnings Per Share [Abstract]

Schedule of Reconciliation of Basic and Diluted Earnings Per Share Computations

The following are computations of basic and diluted earnings per share (in thousands, except for per share amounts):


	For the years ended December 31,
	2023		2022		2021
Numerator for basic and diluted earnings per share:
Net income	$	377,856	$	411,744	$	203,247
Denominator:
Weighted basic shares outstanding	54,071		54,223		54,514
Dilutive effect of equity-based compensation awards	311		264		247
Weighted dilutive shares outstanding	54,382		54,487		54,761

Basic earnings per share	$	6.99	$	7.59	$	3.73
Diluted earnings per share	$	6.95	$	7.56	$	3.71

Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share

The Company included all outstanding performance awards and restricted stock awards in the calculation of diluted earnings per share except for as shown in the table below:


	For the years ended December 31,
	2023	2022	2021
Antidilutive restricted stock awards	12,247	10,250	67,981
Performance stock awards for which performance criteria was not attained at reporting date	88,876	—	14,237