BUSINESS COMBINATION (Details) - USD ($) $ in Thousands		9 Months Ended
	Aug. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
BUSINESS ACQUISITIONS
Payment to acquire business		$ 21,253
Consideration paid and the fair value of assets acquired and liabilities assumed
Goodwill		196,379	$ 189,373
Field and Industrial Services
Consideration paid and the fair value of assets acquired and liabilities assumed
Goodwill		53,056	44,131
Environmental Services
Consideration paid and the fair value of assets acquired and liabilities assumed
Goodwill		150,193	$ 150,699
ES&H Dallas
Consideration paid and the fair value of assets acquired and liabilities assumed
Property and equipment	$ 8,500
Goodwill	8,900
Identifiable intangible assets		$ 3,900
ES&H Dallas | Field and Industrial Services
BUSINESS ACQUISITIONS
Payment to acquire business	21,300
Consideration paid and the fair value of assets acquired and liabilities assumed
Identifiable intangible assets	$ 3,900
Weighted average amortization period	14 years