OPERATING SEGMENTS - Summarized Financial Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
OPERATING SEGMENTS
Number of reportable segments | segment			2
Revenue	$ 151,416	$ 134,054	$ 408,387	$ 370,345
Depreciation, amortization and accretion	10,756	10,776	31,158	31,838
Capital expenditures	10,830	8,802	25,791	26,354
Total assets	853,174	802,651	853,174	802,651	$ 802,076
Corporate
OPERATING SEGMENTS
Depreciation, amortization and accretion	316	118	762	375
Capital expenditures	379	647	1,513	2,250
Total assets	80,957	59,442	80,957	59,442
Environmental Services | Operating Segment
OPERATING SEGMENTS
Revenue	107,197	97,661	292,628	268,555
Depreciation, amortization and accretion	8,903	9,342	26,089	27,238
Capital expenditures	9,878	7,594	20,818	21,204
Total assets	614,228	619,156	614,228	619,156
Field and Industrial Services | Operating Segment
OPERATING SEGMENTS
Revenue	44,219	36,393	115,759	101,790
Depreciation, amortization and accretion	1,537	1,316	4,307	4,225
Capital expenditures	573	561	3,460	2,900
Total assets	$ 157,989	$ 124,053	$ 157,989	$ 124,053