GOODWILL AND INTANGIBLE ASSETS - Goodwill (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2017 USD ($)
Changes in goodwill
Balance at the beginning of the period	$ 193,621
Foreign currency translation	603
Balance at the end of the period	194,224
Environmental Services
Changes in goodwill
Balance at the beginning of the period	149,490
Foreign currency translation	603
Balance at the end of the period	150,093
Field & Industrial Services
Changes in goodwill
Balance at the beginning of the period	44,131
Balance at the end of the period	$ 44,131