BUSINESS SEGMENT INFORMATION - Segment Summary (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Revenues:
Revenues	$ 358,516	$ 467,926	$ 693,172	$ 904,260
Operating income (loss):
Gain on disposition of assets, net	1,133	1,810	4,637	3,173
Total operating income (loss)	8,387	75,029	6,226	135,827
Interest expense	(164)	(113)	(253)	(218)
Interest income	594	458	1,394	860
Other (expense) income , net	(53)	4,104	392	9,499
Income before income taxes	8,764	79,478	7,759	145,968
Depreciation and amortization	42,881	40,094	85,386	77,574
Capital expenditures			132,253	149,692
Identifiable assets	1,223,967	1,218,648	1,223,967	1,218,648	$ 1,199,580
Technical Services
Revenues:
Revenues	338,034	449,852	652,113	868,915
Operating income (loss):
Total operating income (loss)	6,850	75,624	2,392	140,629
Depreciation and amortization			80,370	70,910
Capital expenditures			125,171	144,511
Identifiable assets	1,030,073	983,177	1,030,073	983,177
Support Services
Revenues:
Revenues	20,482	18,074	41,059	35,345
Operating income (loss):
Total operating income (loss)	4,018	1,193	7,155	288
Depreciation and amortization			4,856	6,444
Capital expenditures			6,260	4,467
Identifiable assets	86,928	77,372	86,928	77,372
Corporate
Operating income (loss):
Total operating income (loss)	(3,614)	(3,598)	(7,958)	(8,263)
Depreciation and amortization			160	220
Capital expenditures			822	714
Identifiable assets	$ 106,966	$ 158,099	$ 106,966	$ 158,099