REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	[1]	$ 236,839	$ 269,599
Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		110,846	123,953
Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues		102,716	119,851
Heat Treating
Disaggregation of Revenue [Line Items]
Revenues		14,625	19,532
Other
Disaggregation of Revenue [Line Items]
Revenues		8,652	6,263
United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		194,719	225,429
Other Countries
Disaggregation of Revenue [Line Items]
Revenues		42,120	44,170
IHT
Disaggregation of Revenue [Line Items]
Revenues		107,881	127,056
IHT | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		86,407	102,936
IHT | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues		101	362
IHT | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues		14,146	18,793
IHT | Other
Disaggregation of Revenue [Line Items]
Revenues		7,227	4,965
IHT | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		105,304	123,620
IHT | Other Countries
Disaggregation of Revenue [Line Items]
Revenues		2,577	3,436
MS
Disaggregation of Revenue [Line Items]
Revenues		104,519	121,526
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		102,615	119,489
MS | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues		479	739
MS | Other
Disaggregation of Revenue [Line Items]
Revenues		1,425	1,298
MS | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		76,582	87,452
MS | Other Countries
Disaggregation of Revenue [Line Items]
Revenues		27,937	34,074
Quest Integrity
Disaggregation of Revenue [Line Items]
Revenues		24,439	21,017
Quest Integrity | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		24,439	21,017
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
Disaggregation of Revenue [Line Items]
Revenues		0	0
Quest Integrity | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		12,833	14,357
Quest Integrity | Other Countries
Disaggregation of Revenue [Line Items]
Revenues		$ 11,606	$ 6,660

[1] Revenues attributable to individual countries/geographic areas are based on the country of domicile of the legal entity that performs the work.