GOODWILL AND INTANGIBLE ASSETS - Goodwill (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Changes in goodwill
Balance at the beginning of the period	$ 189,373
Foreign currency translation and other adjustments	(490)
Balance at the end of the period	188,883
Environmental Services
Changes in goodwill
Balance at the beginning of the period	150,699
Foreign currency translation and other adjustments	(490)
Balance at the end of the period	150,209
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