GOODWILL AND OTHER INTANGIBLE ASSETS (Rollforward of Goodwill) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Goodwill [Roll Forward]
Balance at beginning of year	$ 514,189
Increase from current period acquisitions	6,825
Measurement period adjustments from prior period acquisitions	(3,993)
Foreign currency translation	889
Balance at end of year	517,910
Environmental Services
Goodwill [Roll Forward]
Balance at beginning of year	207,019
Increase from current period acquisitions	0
Measurement period adjustments from prior period acquisitions	(2,571)
Foreign currency translation	412
Balance at end of year	204,860
Safety-Kleen
Goodwill [Roll Forward]
Balance at beginning of year	307,170
Increase from current period acquisitions	6,825
Measurement period adjustments from prior period acquisitions	(1,422)
Foreign currency translation	477
Balance at end of year	$ 313,050