GOODWILL AND OTHER INTANGIBLE ASSETS - Rollforward of Goodwill (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($)
Goodwill [Roll Forward]
Balance at January 1, 2019	$ 514,189
Increase from current period acquisitions	12,739
Measurement period adjustments from prior period acquisitions	(4,030)
Foreign currency translation	1,683
Balance at September 30, 2019	524,581
Environmental Services
Goodwill [Roll Forward]
Balance at January 1, 2019	207,019
Increase from current period acquisitions	7,514
Measurement period adjustments from prior period acquisitions	(2,675)
Foreign currency translation	780
Balance at September 30, 2019	212,638
Safety-Kleen
Goodwill [Roll Forward]
Balance at January 1, 2019	307,170
Increase from current period acquisitions	5,225
Measurement period adjustments from prior period acquisitions	(1,355)
Foreign currency translation	903
Balance at September 30, 2019	$ 311,943