SEGMENT REPORTING - Third Party Revenues to Direct Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 891,668	$ 843,181	$ 2,541,185	$ 2,442,099
Corporate Items
Segment Reporting Information [Line Items]
Total revenues	129	467	925	1,148
Corporate Items, net
Segment Reporting Information [Line Items]
Total revenues	(1,478)	(1,151)	(3,316)	(2,574)
Operating and Corporate non-segment
Segment Reporting Information [Line Items]
Total revenues	(1,349)	(684)	(2,391)	(1,426)
Environmental Services
Segment Reporting Information [Line Items]
Total revenues	586,872	542,980	1,658,970	1,570,241
Environmental Services | Operating segments
Segment Reporting Information [Line Items]
Total revenues	550,122	508,813	1,550,114	1,468,417
Environmental Services | Intersegment revenues, net
Segment Reporting Information [Line Items]
Total revenues	35,274	33,022	105,555	99,278
Environmental Services | Corporate Items, net
Segment Reporting Information [Line Items]
Total revenues	1,476	1,145	3,301	2,546
Safety-Kleen
Segment Reporting Information [Line Items]
Total revenues	306,145	300,885	884,606	873,284
Safety-Kleen | Operating segments
Segment Reporting Information [Line Items]
Total revenues	341,417	333,901	990,146	972,534
Safety-Kleen | Intersegment revenues, net
Segment Reporting Information [Line Items]
Total revenues	(35,274)	(33,022)	(105,555)	(99,278)
Safety-Kleen | Corporate Items, net
Segment Reporting Information [Line Items]
Total revenues	$ 2	$ 6	$ 15	$ 28