BUSINESS SEGMENT INFORMATION - Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Revenues:
Revenues	$ 415,858	$ 375,507	$ 892,526	$ 660,131
Operating income:
Operating income	82,369	60,415	173,026	83,450
Pension settlement charges	(911)		(18,286)
Gain on disposition of assets, net	3,015	1,798	5,951	4,752
Interest expense	(73)	(222)	(145)	(400)
Interest income	2,698	128	4,553	143
Other income, net	631	79	1,392	583
Income before income taxes	85,625	60,400	178,826	83,776
Depreciation and amortization	26,203	20,094	50,328	39,560
Capital expenditures			104,488	50,578
Identifiable assets	1,228,188	965,079	1,228,188	965,079	$ 1,129,013
Technical Services
Revenues:
Revenues	390,018	356,103	842,009	622,452
Operating income:
Operating income	77,017	59,827	180,550	81,638
Support Services
Revenues:
Revenues	25,840	19,404	50,517	37,679
Operating income:
Operating income	7,920	3,334	14,564	6,114
Segment reconciling item
Operating income:
Corporate expenses	(4,672)	(4,544)	(9,753)	(9,054)
Pension settlement charges	(911)		(18,286)
Gain on disposition of assets, net	3,015	1,798	5,951	4,752
Depreciation and amortization			25	126
Capital expenditures			1,886	94
Identifiable assets	286,379	186,712	286,379	186,712
Operating Segments
Revenues:
Revenues	415,858	375,507	892,526	660,131
Operating income:
Operating income	82,369	60,415	173,026	83,450
Income before income taxes	85,625	60,400	178,826	83,776
Operating Segments | Technical Services
Operating income:
Depreciation and amortization			45,580	34,682
Capital expenditures			97,317	43,418
Identifiable assets	853,837	702,162	853,837	702,162
Operating Segments | Support Services
Operating income:
Depreciation and amortization			4,723	4,752
Capital expenditures			5,285	7,066
Identifiable assets	$ 87,972	$ 76,205	$ 87,972	$ 76,205