SEGMENT REPORTING - Reconciliation of Third Party Revenues to Direct Revenues (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]
Total revenues	$ 951,479	$ 779,344	$ 2,686,085	$ 2,347,907
Environmental Services
Segment Reporting Information [Line Items]
Total revenues	745,633	650,560	2,124,332	1,968,346
Safety-Kleen Sustainability Solutions
Segment Reporting Information [Line Items]
Total revenues	205,787	128,712	561,536	379,343
Operating Segments | Environmental Services
Segment Reporting Information [Line Items]
Total revenues	743,831	651,689	2,119,856	1,969,445
Operating Segments | Safety-Kleen Sustainability Solutions
Segment Reporting Information [Line Items]
Total revenues	207,589	127,583	566,012	378,244
Intersegment revenues, net | Environmental Services
Segment Reporting Information [Line Items]
Total revenues	1,802	(1,129)	4,476	(1,099)
Intersegment revenues, net | Safety-Kleen Sustainability Solutions
Segment Reporting Information [Line Items]
Total revenues	(1,802)	1,129	(4,476)	1,099
Corporate, non-Segment
Segment Reporting Information [Line Items]
Total revenues	59	72	217	218
Operating and Corporate non-segment
Segment Reporting Information [Line Items]
Total revenues	$ 59	$ 72	$ 217	$ 218