OPERATING SEGMENTS - Summarized Financial Information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
OPERATING SEGMENTS
Number of reportable segments | segment			2
Revenue	$ 136,912	$ 126,057	$ 256,971	$ 236,291
Depreciation, amortization and accretion	10,421	10,684	20,402	21,062
Capital expenditures	7,402	10,333	14,960	17,552
Total assets	824,663	789,446	824,663	789,446	$ 802,076
Corporate
OPERATING SEGMENTS
Depreciation, amortization and accretion	330	128	446	258
Capital expenditures	618	723	1,134	1,603
Total assets	98,160	56,773	98,160	56,773
Environmental Services | Operating Segment
OPERATING SEGMENTS
Revenue	98,960	89,591	185,431	170,894
Depreciation, amortization and accretion	8,676	9,105	17,186	17,895
Capital expenditures	4,935	7,771	10,939	13,610
Total assets	599,706	608,207	599,706	608,207
Field and Industrial Services | Operating Segment
OPERATING SEGMENTS
Revenue	37,952	36,466	71,540	65,397
Depreciation, amortization and accretion	1,415	1,451	2,770	2,909
Capital expenditures	1,849	1,839	2,887	2,339
Total assets	$ 126,797	$ 124,466	$ 126,797	$ 124,466