SEGMENT REPORTING - Adjusted EBITDA Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 135,480	$ 149,825	$ 258,070	$ 251,488
Reconciliation to Consolidated Statements of Operations:
Accretion of environmental liabilities	2,766	2,560	5,327	5,134
Depreciation and amortization	72,494	74,217	147,027	149,572
Income from operations	60,220	73,048	105,716	96,782
Other expense (income), net	500	564	2,865	(2,419)
Loss on sale of businesses	184	0	3,258	0
Interest expense, net of interest income	18,654	20,215	37,441	39,979
Income before provision for income taxes	40,882	52,269	62,152	59,222
Operating segments | Environmental Services
Segment Reporting Information [Line Items]
Adjusted EBITDA	138,083	117,868	246,997	207,378
Operating segments | Safety-Kleen
Segment Reporting Information [Line Items]
Adjusted EBITDA	46,589	79,459	107,737	134,252
Corporate
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ (49,192)	$ (47,502)	$ (96,664)	$ (90,142)