REVENUES - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 710,000	$ 868,678	$ 1,568,563	$ 1,649,517
Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	241,929	275,908	517,202	527,827
Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	127,353	86,722	233,265	158,348
Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	95,128	163,866	242,137	314,613
Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	194,872	216,434	409,353	423,517
Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	50,718	125,748	166,606	225,212
United States
Disaggregation of Revenue [Line Items]
Total revenues	631,629	748,639	1,377,874	1,423,976
Canada
Disaggregation of Revenue [Line Items]
Total revenues	78,371	120,039	190,689	225,541
Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	496,914	563,076	1,063,276	1,072,098
Safety-Kleen
Disaggregation of Revenue [Line Items]
Total revenues	214,556	305,984	507,768	578,461
Operating segments | Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	464,354	526,294	992,458	999,992
Operating segments | Environmental Services | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	241,929	275,908	517,202	527,827
Operating segments | Environmental Services | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	127,353	86,722	233,265	158,348
Operating segments | Environmental Services | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	95,072	163,664	241,991	313,817
Operating segments | Environmental Services | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Environmental Services | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Environmental Services | Oil, Gas and Lodging Services
Disaggregation of Revenue [Line Items]
Total revenues		27,653		61,708
Operating segments | Environmental Services | United States
Disaggregation of Revenue [Line Items]
Total revenues	403,451	431,749	843,465	819,918
Operating segments | Environmental Services | Canada
Disaggregation of Revenue [Line Items]
Total revenues	60,903	94,545	148,993	180,074
Operating segments | Safety-Kleen
Disaggregation of Revenue [Line Items]
Total revenues	245,590	342,182	575,959	648,729
Operating segments | Safety-Kleen | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Safety-Kleen | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Safety-Kleen | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating segments | Safety-Kleen | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	194,872	216,434	409,353	423,517
Operating segments | Safety-Kleen | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	50,718	125,748	166,606	225,212
Operating segments | Safety-Kleen | United States
Disaggregation of Revenue [Line Items]
Total revenues	228,338	316,688	534,871	603,262
Operating segments | Safety-Kleen | Canada
Disaggregation of Revenue [Line Items]
Total revenues	17,252	25,494	41,088	45,467
Corporate
Disaggregation of Revenue [Line Items]
Total revenues	56	202	146	796
Corporate | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Corporate | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Corporate | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	56	202	146	796
Corporate | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Corporate | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Corporate | Oil, Gas and Lodging Services
Disaggregation of Revenue [Line Items]
Total revenues		202		796
Corporate | United States
Disaggregation of Revenue [Line Items]
Total revenues	(160)	202	(462)	796
Corporate | Canada
Disaggregation of Revenue [Line Items]
Total revenues	$ 216	$ 0	$ 608	$ 0