REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 194,618	$ 236,839
Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	87,613	110,846
Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	86,057	102,716
Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	14,144	14,625
Other
Disaggregation of Revenue [Line Items]
Revenues	6,804	8,652
United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	158,326	194,719
Other Countries
Disaggregation of Revenue [Line Items]
Revenues	36,292	42,120
IHT
Disaggregation of Revenue [Line Items]
Revenues	91,139	107,881
IHT | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	71,530	86,407
IHT | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	81	101
IHT | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	13,455	14,146
IHT | Other
Disaggregation of Revenue [Line Items]
Revenues	6,073	7,227
IHT | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	89,225	105,304
IHT | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	1,914	2,577
MS
Disaggregation of Revenue [Line Items]
Revenues	87,396	104,519
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	85,976	102,615
MS | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	689	479
MS | Other
Disaggregation of Revenue [Line Items]
Revenues	731	1,425
MS | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	60,046	76,582
MS | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	27,350	27,937
Quest Integrity
Disaggregation of Revenue [Line Items]
Revenues	16,083	24,439
Quest Integrity | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	16,083	24,439
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	9,055	12,833
Quest Integrity | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	$ 7,028	$ 11,606