REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation of Revenue [Line Items]
Revenues	$ 239,492	$ 221,540	$ 441,769	$ 410,578
Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	94,305	91,701	175,911	168,152
Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	122,241	106,846	222,082	198,640
Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	15,928	15,843	29,934	29,739
Other
Disaggregation of Revenue [Line Items]
Revenues	7,018	7,150	13,842	14,047
IHT
Disaggregation of Revenue [Line Items]
Revenues	116,740	114,124	218,569	209,721
IHT | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	94,305	91,701	175,911	168,152
IHT | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	219	115	222	139
IHT | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	15,717	15,787	29,445	29,626
IHT | Other
Disaggregation of Revenue [Line Items]
Revenues	6,499	6,521	12,991	11,804
MS
Disaggregation of Revenue [Line Items]
Revenues	122,752	107,416	223,200	200,857
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	122,022	106,731	221,860	198,501
MS | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	211	56	489	113
MS | Other
Disaggregation of Revenue [Line Items]
Revenues	519	629	851	2,243
United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	195,639	189,224	366,201	346,533
United States and Canada | IHT
Disaggregation of Revenue [Line Items]
Revenues	113,013	111,541	211,544	204,919
United States and Canada | MS
Disaggregation of Revenue [Line Items]
Revenues	82,626	77,683	154,657	141,614
Other Countries
Disaggregation of Revenue [Line Items]
Revenues	43,853	32,316	75,568	64,045
Other Countries | IHT
Disaggregation of Revenue [Line Items]
Revenues	3,727	2,583	7,025	4,802
Other Countries | MS
Disaggregation of Revenue [Line Items]
Revenues	$ 40,126	$ 29,733	$ 68,543	$ 59,243