REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 290,856	$ 285,067	$ 937,130	$ 883,877
Asset Integrity Management
Disaggregation of Revenue [Line Items]
Revenues	37,314		99,913
Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	120,341		403,598
Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	17,837		60,535
Non-Destructive Evaluation
Disaggregation of Revenue [Line Items]
Revenues	106,175		345,190
Other Services
Disaggregation of Revenue [Line Items]
Revenues	9,189		27,894
United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	240,180		794,861
Other Countries
Disaggregation of Revenue [Line Items]
Revenues	50,676		142,269
IHT
Disaggregation of Revenue [Line Items]
Revenues	147,529		467,621
IHT | Asset Integrity Management
Disaggregation of Revenue [Line Items]
Revenues	12,913		31,054
IHT | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	3,194		9,077
IHT | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	17,019		58,774
IHT | Non-Destructive Evaluation
Disaggregation of Revenue [Line Items]
Revenues	106,175		345,190
IHT | Other Services
Disaggregation of Revenue [Line Items]
Revenues	8,228		23,526
IHT | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	143,508		456,701
IHT | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	4,021		10,920
MS
Disaggregation of Revenue [Line Items]
Revenues	119,011		400,890
MS | Asset Integrity Management
Disaggregation of Revenue [Line Items]
Revenues	85		240
MS | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	117,147		394,521
MS | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	818		1,761
MS | Non-Destructive Evaluation
Disaggregation of Revenue [Line Items]
Revenues	0		0
MS | Other Services
Disaggregation of Revenue [Line Items]
Revenues	961		4,368
MS | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	80,714		292,502
MS | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	38,297		108,388
Quest Integrity
Disaggregation of Revenue [Line Items]
Revenues	24,316		68,619
Quest Integrity | Asset Integrity Management
Disaggregation of Revenue [Line Items]
Revenues	24,316		68,619
Quest Integrity | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	0		0
Quest Integrity | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	0		0
Quest Integrity | Non-Destructive Evaluation
Disaggregation of Revenue [Line Items]
Revenues	0		0
Quest Integrity | Other Services
Disaggregation of Revenue [Line Items]
Revenues	0		0
Quest Integrity | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	15,958		45,658
Quest Integrity | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	$ 8,358		$ 22,961