SEGMENT AND GEOGRAPHIC DISCLOSURES - Segment Data for our Three Operating Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 343,889	$ 312,256	$ 646,274	$ 598,810
Operating income (loss)	1,799	(6,693)	(12,326)	(18,781)
Capital expenditures	6,595	7,944	12,082	18,662
Depreciation and amortization	15,979	13,054	32,434	26,015
IHT
Segment Reporting Information [Line Items]
Revenues	168,673		320,092
MS
Segment Reporting Information [Line Items]
Revenues	148,978		281,879
Quest Integrity
Segment Reporting Information [Line Items]
Revenues	26,238		44,303
Operating segments | IHT
Segment Reporting Information [Line Items]
Revenues	168,673	158,412	320,092	301,368
Operating income (loss)	13,281	10,529	20,021	18,654
Capital expenditures	2,063	2,852	4,398	5,364
Depreciation and amortization	4,725	4,861	9,530	9,716
Operating segments | MS
Segment Reporting Information [Line Items]
Revenues	148,978	132,564	281,879	254,386
Operating income (loss)	10,582	5,385	13,100	5,836
Capital expenditures	2,369	3,580	4,025	7,900
Depreciation and amortization	8,944	5,797	18,222	11,660
Operating segments | Quest Integrity
Segment Reporting Information [Line Items]
Revenues	26,238	21,280	44,303	43,056
Operating income (loss)	5,751	3,889	6,845	8,080
Capital expenditures	1,031	827	1,652	1,273
Depreciation and amortization	966	1,096	1,965	2,365
Corporate and shared support services
Segment Reporting Information [Line Items]
Operating income (loss)	(27,815)	(26,496)	(52,292)	(51,351)
Capital expenditures	1,132	685	2,007	4,125
Depreciation and amortization	$ 1,344	$ 1,300	$ 2,717	$ 2,274