OPERATING SEGMENTS - Reconciliation of EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reconciliation of Net Income to Adjusted EBITDA
Net income	$ 5,049	$ 8,938	$ 10,234	$ 16,455
Income tax expense	2,718	5,866	5,797	10,550
Interest expense	8,474	4,303	12,604	8,862
Interest income	(21)	(33)	(31)	(82)
Foreign currency (gain) loss	(158)	343	(246)	(416)
Other income	(166)	(2,330)	(303)	(2,499)
Depreciation and amortization of plant and equipment	6,987	6,202	13,621	12,106
Amortization of intangibles	2,615	2,646	5,286	5,256
Stock-based compensation	1,043	783	1,959	1,578
Accretion and non-cash adjustment of closure & post-closure liabilities	1,082	1,025	2,155	2,049
Adjusted EBITDA	27,623	27,743	51,076	53,859
Operating Segment | Environmental Services
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	34,642	33,551	66,498	66,604
Operating Segment | Field & Industrial Services
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	4,119	5,123	6,183	8,801
Corporate
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	$ (11,138)	$ (10,931)	$ (21,605)	$ (21,546)