OPERATING SEGMENTS - Reconciliation of EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reconciliation of Net Income to Adjusted EBITDA
Net income	$ 13,220	$ 5,049	$ 22,463	$ 10,234
Income tax expense	4,258	2,718	7,778	5,797
Interest expense	2,907	8,474	5,716	12,604
Interest income	(39)	(21)	(63)	(31)
Foreign currency (gain) loss	139	(158)	153	(246)
Other income	(193)	(166)	(2,316)	(303)
Depreciation and amortization of plant and equipment	7,044	6,987	13,649	13,621
Amortization of intangibles	2,296	2,615	4,598	5,286
Stock-based compensation	1,011	1,043	2,079	1,959
Accretion and non-cash adjustment of closure & post-closure liabilities	1,081	1,082	2,155	2,155
Adjusted EBITDA	31,724	27,623	56,212	51,076
Operating Segment | Environmental Services
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	39,860	34,642	74,532	66,498
Operating Segment | Field and Industrial Services
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	4,562	4,119	6,907	6,183
Corporate
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	$ (12,698)	$ (11,138)	$ (25,227)	$ (21,605)