GOODWILL AND INTANGIBLE ASSETS - Goodwill (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Changes in goodwill
Balance at the beginning of the period	$ 189,373
Foreign currency translation and other adjustments	(894)
Balance at the end of the period	188,479
Environmental Services
Changes in goodwill
Balance at the beginning of the period	150,699
Foreign currency translation and other adjustments	(894)
Balance at the end of the period	149,805
Accumulated Impairment
Accumulated impairment at the beginning	5,457
Accumulated impairment at the ending	(5,457)
Field and Industrial Services
Changes in goodwill
Balance at the beginning of the period	44,131
Balance at the end of the period	$ 44,131