REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	[1]	$ 238,873	$ 189,304	$ 433,491	$ 426,143
Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		116,535	78,973	204,147	189,819
Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues		97,356	92,156	183,413	194,872
Heat Treating
Disaggregation of Revenue [Line Items]
Revenues		17,275	10,966	31,419	25,591
Other
Disaggregation of Revenue [Line Items]
Revenues		7,707	7,209	14,512	15,861
United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		196,617	156,385	354,943	351,103
Other Countries
Disaggregation of Revenue [Line Items]
Revenues		42,256	32,919	78,548	75,040
IHT
Disaggregation of Revenue [Line Items]
Revenues		117,462	80,474	208,601	188,355
IHT | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		92,291	62,963	163,820	149,370
IHT | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues		207	62	288	163
IHT | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues		17,267	10,770	30,722	24,916
IHT | Other
Disaggregation of Revenue [Line Items]
Revenues		7,697	6,679	13,771	13,906
IHT | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		114,834	78,234	204,059	183,536
IHT | Other Countries
Disaggregation of Revenue [Line Items]
Revenues		2,628	2,240	4,542	4,819
MS
Disaggregation of Revenue [Line Items]
Revenues		97,167	92,820	184,563	197,339
MS | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		0	0	0	0
MS | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues		97,149	92,094	183,125	194,709
MS | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues		8	196	697	675
MS | Other
Disaggregation of Revenue [Line Items]
Revenues		10	530	741	1,955
MS | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		65,303	67,403	125,349	143,985
MS | Other Countries
Disaggregation of Revenue [Line Items]
Revenues		31,864	25,417	59,214	53,354
Quest Integrity
Disaggregation of Revenue [Line Items]
Revenues		24,244	16,010	40,327	40,449
Quest Integrity | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		24,244	16,010	40,327	40,449
Quest Integrity | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues		0	0	0	0
Quest Integrity | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues		0	0	0	0
Quest Integrity | Other
Disaggregation of Revenue [Line Items]
Revenues		0	0	0	0
Quest Integrity | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		16,480	10,748	25,535	23,582
Quest Integrity | Other Countries
Disaggregation of Revenue [Line Items]
Revenues		$ 7,764	$ 5,262	$ 14,792	$ 16,867

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