OPERATING SEGMENTS - Reconciliation of EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reconciliation of Net Income to Adjusted EBITDA
Net income	$ 9,243	$ 5,185
Income tax (benefit) expense	3,520	3,079
Interest expense	2,809	4,130
Interest income	(24)	(10)
Foreign currency (gain) loss	14	(88)
Other income	(2,123)	(137)
Depreciation and amortization of plant and equipment	6,605	6,633
Amortization of intangibles	2,302	2,670
Stock-based compensation	1,068	918
Accretion and non-cash adjustment of closure & post-closure liabilities	1,074	1,073
Adjusted EBITDA	24,488	23,453
Operating Segment | Environmental Services
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	34,672	31,856
Operating Segment | Field and Industrial Services
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	2,345	2,064
Corporate
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	$ (12,529)	$ (10,467)