OPERATING SEGMENTS - Reconciliation of EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reconciliation of Net Income to Adjusted EBITDA
Net income	$ 15,491	$ 13,220	$ 23,534	$ 22,463
Income tax expense	6,395	4,258	9,436	7,778
Interest expense	3,588	2,907	7,618	5,716
Interest income	(202)	(39)	(409)	(63)
Foreign currency loss	384	139	523	153
Other income	(122)	(193)	(232)	(2,316)
Property and equipment impairment charges			25
Depreciation and amortization of plant and equipment	9,129	7,044	17,254	13,649
Amortization of intangibles assets	2,863	2,296	5,674	4,598
Share-based compensation	1,245	1,011	2,467	2,079
Accretion and non-cash adjustment of closure & post-closure liabilities	1,133	1,081	2,258	2,155
Property insurance recoveries	(4,500)		(9,153)
Adjusted EBITDA	35,404	31,724	58,995	56,212
Operating Segment | Environmental Services
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	47,056	39,860	82,316	74,532
Operating Segment | Field and Industrial Services
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	5,022	4,562	7,576	6,907
Corporate
Reconciliation of Net Income to Adjusted EBITDA
Adjusted EBITDA	$ (16,674)	$ (12,698)	$ (30,897)	$ (25,227)