GOODWILL AND OTHER INTANGIBLE ASSETS - Rollforward of Goodwill (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)
Goodwill [Roll Forward]
Balance at January 1, 2019	$ 514,189
Increase from current period acquisitions	12,536
Measurement period adjustments from prior period acquisitions	(4,032)
Foreign currency translation	2,351
Balance at June 30, 2019	525,044
Environmental Services
Goodwill [Roll Forward]
Balance at January 1, 2019	207,019
Increase from current period acquisitions	7,311
Measurement period adjustments from prior period acquisitions	(2,611)
Foreign currency translation	1,089
Balance at June 30, 2019	212,808
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	(1,421)
Foreign currency translation	1,262
Balance at June 30, 2019	$ 312,236