REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	[1]	$ 290,079	$ 290,856	$ 875,507	$ 937,130
Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		130,523	143,489	394,741	445,103
Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues		134,196	120,341	397,091	403,598
Heat Treating
Disaggregation of Revenue [Line Items]
Revenues		16,882	17,837	54,845	60,535
Other
Disaggregation of Revenue [Line Items]
Revenues		8,478	9,189	28,830	27,894
United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		242,071	240,180	733,393	794,861
Other Countries
Disaggregation of Revenue [Line Items]
Revenues		48,008	50,676	142,114	142,269
IHT
Disaggregation of Revenue [Line Items]
Revenues		126,379	147,529	392,093	467,621
IHT | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		102,448	119,088	313,375	376,244
IHT | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues		100	3,194	598	9,077
IHT | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues		16,500	17,019	52,804	58,774
IHT | Other
Disaggregation of Revenue [Line Items]
Revenues		7,331	8,228	25,316	23,526
IHT | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		121,772	143,508	379,460	456,701
IHT | Other Countries
Disaggregation of Revenue [Line Items]
Revenues		4,607	4,021	12,633	10,920
MS
Disaggregation of Revenue [Line Items]
Revenues		135,625	119,011	402,048	400,890
MS | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		0	85	0	240
MS | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues		134,096	117,147	396,493	394,521
MS | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues		382	818	2,041	1,761
MS | Other
Disaggregation of Revenue [Line Items]
Revenues		1,147	961	3,514	4,368
MS | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues		101,929	80,714	296,557	292,502
MS | Other Countries
Disaggregation of Revenue [Line Items]
Revenues		33,696	38,297	105,491	108,388
Quest Integrity
Disaggregation of Revenue [Line Items]
Revenues		28,075	24,316	81,366	68,619
Quest Integrity | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues		28,075	24,316	81,366	68,619
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		18,370	15,958	57,376	45,658
Quest Integrity | Other Countries
Disaggregation of Revenue [Line Items]
Revenues		$ 9,705	$ 8,358	$ 23,990	$ 22,961

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