GOODWILL AND INTANGIBLE ASSETS - Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Intangible Assets
Amortizing intangible assets, Cost	$ 218,856		$ 218,856		$ 216,622
Accumulated amortization	(48,658)		(48,658)		(42,186)
Amortizing intangible assets, Net	170,198		170,198		174,436
Total intangible assets, cost	265,412		265,412		264,998
Total intangible assets, net	216,754		216,754		222,812
Amortization of intangible assets	2,327	$ 2,300	6,925	$ 7,586
Permits and licenses
Intangible Assets
Nonamortizing intangible assets	46,424		46,424		48,241
Tradename
Intangible Assets
Nonamortizing intangible assets	132		132		135
Permits, licenses and lease
Intangible Assets
Amortizing intangible assets, Cost	111,200		111,200		111,818
Accumulated amortization	(14,324)		(14,324)		(12,459)
Amortizing intangible assets, Net	96,876		96,876		99,359
Customer relationships
Intangible Assets
Amortizing intangible assets, Cost	87,751		87,751		84,977
Accumulated amortization	(24,158)		(24,158)		(20,168)
Amortizing intangible assets, Net	63,593		63,593		64,809
Technology - formulae and processes
Intangible Assets
Amortizing intangible assets, Cost	7,049		7,049		7,250
Accumulated amortization	(1,755)		(1,755)		(1,630)
Amortizing intangible assets, Net	5,294		5,294		5,620
Customer backlog
Intangible Assets
Amortizing intangible assets, Cost	3,652		3,652		3,652
Accumulated amortization	(1,565)		(1,565)		(1,291)
Amortizing intangible assets, Net	2,087		2,087		2,361
Tradename
Intangible Assets
Amortizing intangible assets, Cost	4,290		4,290		4,318
Accumulated amortization	(4,290)		(4,290)		(4,318)
Developed software
Intangible Assets
Amortizing intangible assets, Cost	2,898		2,898		2,926
Accumulated amortization	(1,522)		(1,522)		(1,319)
Amortizing intangible assets, Net	1,376		1,376		1,607
Non-compete agreements
Intangible Assets
Amortizing intangible assets, Cost	1,092		1,092		748
Accumulated amortization	(757)		(757)		(748)
Amortizing intangible assets, Net	335		335
Internet domain and website
Intangible Assets
Amortizing intangible assets, Cost	536		536		540
Accumulated amortization	(121)		(121)		(100)
Amortizing intangible assets, Net	415		415		440
Database
Intangible Assets
Amortizing intangible assets, Cost	388		388		393
Accumulated amortization	(166)		(166)		(153)
Amortizing intangible assets, Net	222		222		$ 240
ES&H Dallas
Intangible Assets
Identifiable intangible assets	$ 3,900		$ 3,900