CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CONSOLIDATED STATEMENTS OF OPERATIONS
Net revenues	$ 300,891	$ 279,638	$ 281,098	$ 236,419	$ 284,853	$ 262,946	$ 266,966	$ 233,995	$ 1,098,046	$ 1,048,760	$ 1,040,769
Material costs	95,961	92,034	91,399	78,377	90,340	84,805	86,516	76,356	357,771	338,017	329,223
Personnel costs	99,430	94,594	91,490	86,711	97,574	90,853	89,554	86,108	372,225	364,089	361,090
Other operating costs	34,876	32,771	33,741	33,555	31,271	30,999	30,536	31,096	134,943	123,902	129,831
General and administrative expenses	30,591	29,834	29,358	28,282	29,085	28,308	26,523	25,636	118,065	109,552	109,342
Professional accounting and legal fees	4,113	3,629	3,247	2,700	4,726	3,107	4,236	4,846	13,689	16,915	36,239
Depreciation and amortization	9,019	9,373	8,760	8,773	8,903	8,950	9,272	9,330	35,925	36,455	39,259
Impairment of intangible assets					183					183	54,735
Income (loss) from operations	26,901	17,403	23,103	(1,979)	22,771	15,924	20,329	623	65,428	59,647	(18,950)
Interest expense, net	8,285	8,954	8,481	8,538	9,046	8,939	7,317	12,263	34,258	37,566	57,688
Loss on extinguishment of debt								16,998		16,998
Non-service defined benefit plan expense	172	173	173	173	176	176	176	176	691	703	736
Income (loss) before income taxes	18,444	8,276	14,449	(10,690)	13,549	6,809	12,836	(28,814)	30,479	4,380	(77,374)
Provision for income taxes	(306)	2,585	4,414	(3,739)	9,086	2,440	(92)	(6,196)	2,954	5,238	27,297
Net income (loss)	$ 18,750	$ 5,691	$ 10,035	$ (6,951)	$ 4,463	$ 4,369	$ 12,928	$ (22,618)	$ 27,525	$ (858)	$ (104,671)
Basic and Diluted Per Common Share Data:
Basic income (loss) per share (in dollars per share)	$ 0.50	$ 0.15	$ 0.27	$ (0.19)	$ 0.12	$ 0.12	$ 0.35	$ (0.62)	$ 0.74	$ (0.02)	$ (2.89)
Weighted average shares used to compute basic earnings per common share	37,411,847	37,349,144	37,299,766	37,001,977	36,906,938	36,856,881	36,790,401	36,498,482	37,267,188	36,764,551	36,270,920
Diluted income (loss) per share	$ 0.49	$ 0.15	$ 0.26	$ (0.19)	$ 0.12	$ 0.12	$ 0.35	$ (0.62)	$ 0.72	$ (0.02)	$ (2.89)
Weighted average shares used to compute diluted earnings per common share	38,415,108	37,986,860	37,887,559	37,001,977	37,721,662	37,556,594	37,404,360	36,498,482	38,064,617	36,764,551	36,270,920