REVENUES - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Total revenues	$ 926,458	$ 710,000	$ 1,734,606	$ 1,568,563
Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	306,865	241,929	578,905	517,202
Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	106,986	127,353	212,154	233,265
Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	150,446	95,128	270,335	242,137
Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	199,382	194,872	394,220	409,353
Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	162,779	50,718	278,992	166,606
United States
Disaggregation of Revenue [Line Items]
Total revenues	796,880	631,629	1,511,457	1,377,874
Canada
Disaggregation of Revenue [Line Items]
Total revenues	129,578	78,371	223,149	190,689
Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	724,097	612,594	1,378,699	1,317,786
Safety-Kleen Sustainability Solutions
Disaggregation of Revenue [Line Items]
Total revenues	202,282	97,350	355,749	250,631
Operating segments | Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	723,147	612,720	1,376,025	1,317,756
Operating segments | Environmental Services | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	306,865	241,929	578,905	517,202
Operating segments | Environmental Services | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	106,986	127,353	212,154	233,265
Operating segments | Environmental Services | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	150,367	95,072	270,177	241,991
Operating segments | Environmental Services | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	158,929	148,366	314,789	325,298
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	616,585	543,785	1,192,093	1,150,895
Operating segments | Environmental Services | Canada
Disaggregation of Revenue [Line Items]
Total revenues	106,562	68,935	183,932	166,861
Operating segments | Safety-Kleen Sustainability Solutions
Disaggregation of Revenue [Line Items]
Total revenues	203,232	97,224	358,423	250,661
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	40,453	46,506	79,431	84,055
Operating segments | Safety-Kleen Sustainability Solutions | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	162,779	50,718	278,992	166,606
Operating segments | Safety-Kleen Sustainability Solutions | United States
Disaggregation of Revenue [Line Items]
Total revenues	180,216	88,004	319,206	227,441
Operating segments | Safety-Kleen Sustainability Solutions | Canada
Disaggregation of Revenue [Line Items]
Total revenues	23,016	9,220	39,217	23,220
Corporate, non-Segment
Disaggregation of Revenue [Line Items]
Total revenues	79	56	158	146
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	79	56	158	146
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	79	(160)	158	(462)
Corporate, non-Segment | Canada
Disaggregation of Revenue [Line Items]
Total revenues	$ 0	$ 216	$ 0	$ 608