REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 315,829	$ 343,889	$ 585,428	$ 646,274
Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	140,265	160,013	264,218	301,614
Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	143,044	151,212	262,895	283,257
Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	18,431	22,898	37,963	42,698
Other Services
Disaggregation of Revenue [Line Items]
Revenues	14,089	9,766	20,352	18,705
United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	265,893	293,937	491,322	554,681
Other Countries
Disaggregation of Revenue [Line Items]
Revenues	49,936	49,952	94,106	91,593
IHT
Disaggregation of Revenue [Line Items]
Revenues	138,658	168,673	265,714	320,092
IHT | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	107,991	133,620	210,927	257,156
IHT | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	136	5,037	498	5,883
IHT | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	17,511	22,459	36,304	41,755
IHT | Other Services
Disaggregation of Revenue [Line Items]
Revenues	13,020	7,557	17,985	15,298
IHT | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	134,068	164,983	257,688	313,193
IHT | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	4,590	3,690	8,026	6,899
MS
Disaggregation of Revenue [Line Items]
Revenues	144,897	148,978	266,423	281,879
MS | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	0	155	0	155
MS | Repair and Maintenance Services
Disaggregation of Revenue [Line Items]
Revenues	142,908	146,175	262,397	277,374
MS | Heat Treating
Disaggregation of Revenue [Line Items]
Revenues	920	439	1,659	943
MS | Other Services
Disaggregation of Revenue [Line Items]
Revenues	1,069	2,209	2,367	3,407
MS | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	107,176	111,964	194,628	211,788
MS | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	37,721	37,014	71,795	70,091
Quest Integrity
Disaggregation of Revenue [Line Items]
Revenues	32,274	26,238	53,291	44,303
Quest Integrity | Non-Destructive Evaluation and Testing Services
Disaggregation of Revenue [Line Items]
Revenues	32,274	26,238	53,291	44,303
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 Services
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Quest Integrity | United States and Canada
Disaggregation of Revenue [Line Items]
Revenues	24,649	16,990	39,006	29,700
Quest Integrity | Other Countries
Disaggregation of Revenue [Line Items]
Revenues	$ 7,625	$ 9,248	$ 14,285	$ 14,603