SEGMENT REPORTING - Reconciliation to Consolidated Statements of Income to Adjusted EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 185,084	$ 167,833	$ 502,306	$ 431,980
Reconciliation of Net Income to Adjusted EBITDA [Abstract]
Accretion of environmental liabilities	2,799	2,822	8,625	8,149
Stock-based compensation	6,001	6,662	12,786	12,739
Depreciation and amortization	71,451	74,470	215,206	221,497
Income from operations	104,833	83,879	265,689	189,595
Other (income) expense, net	(199)	(2,268)	2,509	597
Loss on sale of businesses	0	118	0	3,376
Interest expense, net of interest income	17,984	17,407	53,953	54,848
Income before provision for income taxes	87,048	68,622	209,227	130,774
Operating segments | Environmental Services
Segment Reporting Information [Line Items]
Adjusted EBITDA	166,471	180,002	482,766	502,101
Operating segments | Safety-Kleen Sustainability Solutions
Segment Reporting Information [Line Items]
Adjusted EBITDA	70,810	29,613	165,756	62,248
Corporate, non-Segment
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ (52,197)	$ (41,782)	$ (146,216)	$ (132,369)