REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 219,093	$ 290,079	$ 645,236	[1]	$ 875,507	[1]
Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	98,041	130,523	287,860		394,741
Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	101,032	134,196	295,862		397,091
Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	12,888	16,882	38,479		54,845
Other
Disaggregation of Revenue [Line Items]
Revenues	7,132	8,478	23,035		28,830
United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	180,000	242,071	531,103		733,393
Other Countries
Disaggregation of Revenue [Line Items]
Revenues	39,093	48,008	114,133		142,114
IHT
Disaggregation of Revenue [Line Items]
Revenues	96,637	126,379	284,992		392,093
IHT | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	77,323	102,448	226,693		313,375
IHT | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	0	100	121		598
IHT | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	12,667	16,500	37,583		52,804
IHT | Other
Disaggregation of Revenue [Line Items]
Revenues	6,647	7,331	20,595		25,316
IHT | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	94,414	121,772	277,947		379,460
IHT | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	2,223	4,607	7,045		12,633
MS
Disaggregation of Revenue [Line Items]
Revenues	101,738	135,625	299,077		402,048
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	101,032	134,096	295,741		396,493
MS | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	221	382	896		2,041
MS | Other
Disaggregation of Revenue [Line Items]
Revenues	485	1,147	2,440		3,514
MS | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	73,204	101,929	217,189		296,557
MS | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	28,534	33,696	81,888		105,491
Quest Integrity
Disaggregation of Revenue [Line Items]
Revenues	20,718	28,075	61,167		81,366
Quest Integrity | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	20,718	28,075	61,167		81,366
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	12,382	18,370	35,967		57,376
Quest Integrity | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	$ 8,336	$ 9,705	$ 25,200		$ 23,990

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