GOODWILL AND INTANGIBLE ASSETS - Goodwill (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)
Changes in goodwill
Balance at the beginning of the period	$ 207,177
Foreign currency translation	703
Balance at the end of the period	210,466
Winnie
Changes in goodwill
Balance at the beginning of the period	13,573
Purchase price allocation adjustment	2,586
Balance at the end of the period	16,159
Environmental Services
Changes in goodwill
Balance at the beginning of the period	162,816
Foreign currency translation	703
Balance at the end of the period	166,105
Accumulated Impairment
Accumulated impairment at the beginning	(6,870)
Accumulated impairment at the ending	(6,870)
Environmental Services | Winnie
Changes in goodwill
Purchase price allocation adjustment	2,586
Field and Industrial Services
Changes in goodwill
Balance at the beginning of the period	51,231
Balance at the end of the period	$ 51,231