REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 269,599	$ 302,385
Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	123,953	141,601
Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	119,851	132,045
Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	19,532	19,800
Other Services
Disaggregation of Revenue [Line Items]
Revenues	6,263	8,939
United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	225,429	260,744
Other Countries
Disaggregation of Revenue [Line Items]
Revenues	44,170	41,641
IHT
Disaggregation of Revenue [Line Items]
Revenues	127,056	151,419
IHT | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	102,936	123,536
IHT | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	362	846
IHT | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	18,793	19,296
IHT | Other Services
Disaggregation of Revenue [Line Items]
Revenues	4,965	7,741
IHT | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	123,620	148,210
IHT | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	3,436	3,209
MS
Disaggregation of Revenue [Line Items]
Revenues	121,526	132,901
MS | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	0	0
MS | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	119,489	131,199
MS | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	739	504
MS | Other Services
Disaggregation of Revenue [Line Items]
Revenues	1,298	1,198
MS | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	87,452	99,824
MS | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	34,074	33,077
Quest Integrity
Disaggregation of Revenue [Line Items]
Revenues	21,017	18,065
Quest Integrity | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	21,017	18,065
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 | Other Services
Disaggregation of Revenue [Line Items]
Revenues	0	0
Quest Integrity | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	14,357	12,710
Quest Integrity | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	$ 6,660	$ 5,355