REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation of Revenue [Line Items]
Revenues	$ 206,715	$ 218,339	$ 648,484	$ 628,917
Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	83,207	87,267	259,118	255,419
Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	101,663	106,795	323,745	305,435
Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	13,001	16,482	42,935	46,221
Other
Disaggregation of Revenue [Line Items]
Revenues	8,844	7,795	22,686	21,842
IHT
Disaggregation of Revenue [Line Items]
Revenues	103,857	110,312	322,426	320,033
IHT | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	83,207	87,267	259,118	255,419
IHT | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	39	19	261	158
IHT | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	12,946	16,357	42,391	45,983
IHT | Other
Disaggregation of Revenue [Line Items]
Revenues	7,665	6,669	20,656	18,473
MS
Disaggregation of Revenue [Line Items]
Revenues	102,858	108,027	326,058	308,884
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	101,624	106,776	323,484	305,277
MS | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	55	125	544	238
MS | Other
Disaggregation of Revenue [Line Items]
Revenues	1,179	1,126	2,030	3,369
United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	168,086	184,144	534,287	530,677
United States and Canada | IHT
Disaggregation of Revenue [Line Items]
Revenues	100,800	108,009	312,344	312,928
United States and Canada | MS
Disaggregation of Revenue [Line Items]
Revenues	67,286	76,135	221,943	217,749
Other Countries
Disaggregation of Revenue [Line Items]
Revenues	38,629	34,195	114,197	98,240
Other Countries | IHT
Disaggregation of Revenue [Line Items]
Revenues	3,057	2,303	10,082	7,105
Other Countries | MS
Disaggregation of Revenue [Line Items]
Revenues	$ 35,572	$ 31,892	$ 104,115	$ 91,135