REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	[1]	$ 217,410	$ 219,093	$ 650,901	$ 645,236
Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		100,084	98,041	304,232	287,860
Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues		95,616	101,032	279,029	295,862
Heat Treating
Disaggregation of Revenue [Line Items]
Revenues		11,999	12,888	43,418	38,479
Other
Disaggregation of Revenue [Line Items]
Revenues		9,711	7,132	24,222	23,035
United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		174,118	180,000	529,061	531,103
Other Countries
Disaggregation of Revenue [Line Items]
Revenues		43,292	39,093	121,840	114,133
IHT
Disaggregation of Revenue [Line Items]
Revenues		101,476	96,637	310,077	284,992
IHT | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		80,553	77,323	244,374	226,693
IHT | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues		56	0	344	121
IHT | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues		11,928	12,667	42,650	37,583
IHT | Other
Disaggregation of Revenue [Line Items]
Revenues		8,939	6,647	22,709	20,595
IHT | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		98,812	94,414	302,871	277,947
IHT | Other Countries
Disaggregation of Revenue [Line Items]
Revenues		2,664	2,223	7,206	7,045
MS
Disaggregation of Revenue [Line Items]
Revenues		96,403	101,738	280,966	299,077
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		95,560	101,032	278,685	295,741
MS | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues		71	221	768	896
MS | Other
Disaggregation of Revenue [Line Items]
Revenues		772	485	1,513	2,440
MS | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		63,885	73,204	189,234	217,189
MS | Other Countries
Disaggregation of Revenue [Line Items]
Revenues		32,518	28,534	91,732	81,888
Quest Integrity
Disaggregation of Revenue [Line Items]
Revenues		19,531	20,718	59,858	61,167
Quest Integrity | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		19,531	20,718	59,858	61,167
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		11,421	12,382	36,956	35,967
Quest Integrity | Other Countries
Disaggregation of Revenue [Line Items]
Revenues		$ 8,110	$ 8,336	$ 22,902	$ 25,200

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