REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020		Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 189,304	$ 315,829	$ 426,143	[1]	$ 585,428	[1]
Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	78,973	140,265	189,819		264,218
Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	92,156	143,044	194,872		262,895
Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	10,966	18,431	25,591		37,963
Other
Disaggregation of Revenue [Line Items]
Revenues	7,209	14,089	15,861		20,352
United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	156,385	265,893	351,103		491,322
Other Countries
Disaggregation of Revenue [Line Items]
Revenues	32,919	49,936	75,040		94,106
IHT
Disaggregation of Revenue [Line Items]
Revenues	80,474	138,658	188,355		265,714
IHT | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	62,963	107,991	149,370		210,927
IHT | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	62	136	163		498
IHT | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	10,770	17,511	24,916		36,304
IHT | Other
Disaggregation of Revenue [Line Items]
Revenues	6,679	13,020	13,906		17,985
IHT | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	78,234	134,068	183,536		257,688
IHT | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	2,240	4,590	4,819		8,026
MS
Disaggregation of Revenue [Line Items]
Revenues	92,820	144,897	197,339		266,423
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	92,094	142,908	194,709		262,397
MS | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	196	920	675		1,659
MS | Other
Disaggregation of Revenue [Line Items]
Revenues	530	1,069	1,955		2,367
MS | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	67,403	107,176	143,985		194,628
MS | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	25,417	37,721	53,354		71,795
Quest Integrity
Disaggregation of Revenue [Line Items]
Revenues	16,010	32,274	40,449		53,291
Quest Integrity | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	16,010	32,274	40,449		53,291
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	10,748	24,649	23,582		39,006
Quest Integrity | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	$ 5,262	$ 7,625	$ 16,867		$ 14,285

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