GOODWILL AND OTHER INTANGIBLE ASSETS (Rollforward of Goodwill) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Goodwill [Roll Forward]
Balance at beginning of year	$ 478,523
Increase from current period acquisitions	37,449
Measurement period adjustments from prior period acquisitions	(78)
Foreign currency translation	(1,792)
Balance at end of year	514,102
Environmental Services
Goodwill [Roll Forward]
Balance at beginning of year	172,386
Increase from current period acquisitions	33,281
Measurement period adjustments from prior period acquisitions	(78)
Foreign currency translation	(829)
Balance at end of year	204,760
Safety-Kleen
Goodwill [Roll Forward]
Balance at beginning of year	306,137
Increase from current period acquisitions	4,168
Measurement period adjustments from prior period acquisitions	0
Foreign currency translation	(963)
Balance at end of year	$ 309,342