CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Net revenues	$ 289,827	$ 256,637	$ 808,116	$ 723,810
Material costs	92,561	81,462	257,002	228,675
Personnel costs	98,948	89,727	286,377	252,734
Other operating costs	34,871	29,979	99,157	74,207
General and administrative expenses	29,620	33,591	93,633	98,918
Depreciation and amortization	8,159	8,803	24,164	26,513
Income from operations	25,668	13,075	47,783	42,763
Interest expense, net	7,313	8,013	21,805	24,918
Non-service defined benefit plan expense	167	158	501	474
Income before income taxes	18,188	4,904	25,477	17,371
Benefit for income taxes	(2,929)	(1,911)	(2,469)	(4,750)
Net income	$ 21,117	$ 6,815	$ 27,946	$ 22,121
Basic and Diluted Per Common Share Data:
Basic income per share (in dollars per share)	$ 0.55	$ 0.18	$ 0.72	$ 0.58
Weighted average shares used to compute basic earnings per common share (in shares)	38,730,468	38,133,598	38,550,307	37,878,753
Diluted income per share (in dollars per share)	$ 0.54	$ 0.18	$ 0.71	$ 0.57
Weighted average shares used to compute diluted earnings per common share (in shares)	39,240,974	38,637,536	39,238,370	38,491,965