REVENUES - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Total revenues	$ 951,479	$ 779,344	$ 2,686,085	$ 2,347,907
Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	308,158	257,612	887,063	774,814
Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	116,677	115,989	328,831	349,254
Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	155,036	124,654	425,371	366,791
Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	203,570	203,455	597,790	612,808
Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	168,038	77,634	447,030	244,240
United States
Disaggregation of Revenue [Line Items]
Total revenues	817,910	666,649	2,329,367	2,044,523
Canada
Disaggregation of Revenue [Line Items]
Total revenues	133,569	112,695	356,718	303,384
Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	745,633	650,560	2,124,332	1,968,346
Safety-Kleen Sustainability Solutions
Disaggregation of Revenue [Line Items]
Total revenues	205,787	128,712	561,536	379,343
Operating segments | Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	743,831	651,689	2,119,856	1,969,445
Operating segments | Environmental Services | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	308,158	257,612	887,063	774,814
Operating segments | Environmental Services | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	116,677	115,989	328,831	349,254
Operating segments | Environmental Services | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	154,977	124,582	425,154	366,573
Operating segments | Environmental Services | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	164,019	153,506	478,808	478,804
Operating segments | Environmental Services | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Environmental Services | United States
Disaggregation of Revenue [Line Items]
Total revenues	632,755	554,050	1,824,848	1,704,945
Operating segments | Environmental Services | Canada
Disaggregation of Revenue [Line Items]
Total revenues	111,076	97,639	295,008	264,500
Operating segments | Safety-Kleen Sustainability Solutions
Disaggregation of Revenue [Line Items]
Total revenues	207,589	127,583	566,012	378,244
Operating segments | Safety-Kleen Sustainability Solutions | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Safety-Kleen Sustainability Solutions | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Safety-Kleen Sustainability Solutions | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Safety-Kleen Sustainability Solutions | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	39,551	49,949	118,982	134,004
Operating segments | Safety-Kleen Sustainability Solutions | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	168,038	77,634	447,030	244,240
Operating segments | Safety-Kleen Sustainability Solutions | United States
Disaggregation of Revenue [Line Items]
Total revenues	185,096	112,785	504,302	340,226
Operating segments | Safety-Kleen Sustainability Solutions | Canada
Disaggregation of Revenue [Line Items]
Total revenues	22,493	14,798	61,710	38,018
Corporate, non-Segment
Disaggregation of Revenue [Line Items]
Total revenues	59	72	217	218
Corporate, non-Segment | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Corporate, non-Segment | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Corporate, non-Segment | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	59	72	217	218
Corporate, non-Segment | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Corporate, non-Segment | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Corporate, non-Segment | United States
Disaggregation of Revenue [Line Items]
Total revenues	59	(186)	217	(648)
Corporate, non-Segment | Canada
Disaggregation of Revenue [Line Items]
Total revenues	$ 0	$ 258	$ 0	$ 866