REVENUES (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Total revenues	$ 808,148	$ 858,563
Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	272,040	275,273
Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	105,168	105,912
Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	119,889	147,009
Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	194,838	214,481
Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	116,213	115,888
United States
Disaggregation of Revenue [Line Items]
Total revenues	714,577	746,245
Canada
Disaggregation of Revenue [Line Items]
Total revenues	93,571	112,318
Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	654,602	705,192
Safety-Kleen Sustainability Solutions
Disaggregation of Revenue [Line Items]
Total revenues	153,467	153,281
Operating segments | Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	652,878	705,036
Operating segments | Environmental Services | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	272,040	275,273
Operating segments | Environmental Services | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	105,168	105,912
Operating segments | Environmental Services | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	119,810	146,919
Operating segments | Environmental Services | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	155,860	176,932
Operating segments | Environmental Services | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Operating segments | Environmental Services | United States
Disaggregation of Revenue [Line Items]
Total revenues	575,508	607,110
Operating segments | Environmental Services | Canada
Disaggregation of Revenue [Line Items]
Total revenues	77,370	97,926
Operating segments | Safety-Kleen Sustainability Solutions
Disaggregation of Revenue [Line Items]
Total revenues	155,191	153,437
Operating segments | Safety-Kleen Sustainability Solutions | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Operating segments | Safety-Kleen Sustainability Solutions | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Operating segments | Safety-Kleen Sustainability Solutions | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Operating segments | Safety-Kleen Sustainability Solutions | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	38,978	37,549
Operating segments | Safety-Kleen Sustainability Solutions | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	116,213	115,888
Operating segments | Safety-Kleen Sustainability Solutions | United States
Disaggregation of Revenue [Line Items]
Total revenues	138,990	139,437
Operating segments | Safety-Kleen Sustainability Solutions | Canada
Disaggregation of Revenue [Line Items]
Total revenues	16,201	14,000
Corporate, non-Segment
Disaggregation of Revenue [Line Items]
Total revenues	79	90
Corporate, non-Segment | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Corporate, non-Segment | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Corporate, non-Segment | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	79	90
Corporate, non-Segment | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Corporate, non-Segment | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Corporate, non-Segment | United States
Disaggregation of Revenue [Line Items]
Total revenues	79	(302)
Corporate, non-Segment | Canada
Disaggregation of Revenue [Line Items]
Total revenues	$ 0	$ 392