GOODWILL AND OTHER INTANGIBLE ASSETS (Changes to Goodwill) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Goodwill, beginning of period	$ 514,189	$ 478,523
Increase from current period acquisitions	12,603	40,704
Measurement period adjustments from prior period acquisitions	(4,030)	(78)
Decrease from disposition of business		0
Foreign currency translation	2,251	(4,960)
Goodwill, end of period	525,013	514,189
Environmental Services
Goodwill [Roll Forward]
Goodwill, beginning of period	207,019	172,386
Increase from current period acquisitions	7,378	37,007
Measurement period adjustments from prior period acquisitions	(2,675)	(78)
Decrease from disposition of business		0
Foreign currency translation	809	(2,296)
Goodwill, end of period	212,531	207,019
Safety-Kleen
Goodwill [Roll Forward]
Goodwill, beginning of period	307,170	306,137
Increase from current period acquisitions	5,225	3,697
Measurement period adjustments from prior period acquisitions	(1,355)	0
Decrease from disposition of business		0
Foreign currency translation	1,442	(2,664)
Goodwill, end of period	$ 312,482	$ 307,170