OPERATING SEGMENTS - Reconciliation of EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reconciliation of Net Income to Adjusted EBITDA
Net income	$ 8,365	$ 10,114	$ 18,599	$ 26,569
Income tax expense	4,668	6,278	10,465	16,828
Interest expense	2,783	4,288	15,387	13,150
Interest income	(18)	(8)	(49)	(90)
Foreign currency (gain) loss	(275)	224	(521)	(192)
Other income	(234)	19	(537)	(2,480)
Depreciation and amortization of plant and equipment	7,386	6,454	21,007	18,561
Amortization of intangibles	2,300	2,651	7,586	7,907
Stock-based compensation	995	605	2,954	2,182
Accretion and non-cash adjustment of closure & post-closure liabilities	1,090	1,031	3,245	3,081
Adjusted EBITDA	27,060	31,656	78,136	85,516
Operating Segment | Environmental Services
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	35,524	37,747	102,022	104,352
Operating Segment | Field & Industrial Services
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	3,646	4,466	9,829	13,267
Corporate
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	$ (12,110)	$ (10,557)	$ (33,715)	$ (32,103)