Unaudited Quarterly Financial Data - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended																12 Months Ended
		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information
Total revenues		$ 195,169		$ 189,311		$ 190,252		$ 186,226		$ 197,288		$ 186,960		$ 182,641		$ 185,631		$ 760,958	$ 752,520	$ 751,286
Total cost of revenues (exclusive of depreciation)		99,461		93,420		92,783		89,483		95,497		92,182		91,796		100,266		375,147	379,741	389,343
Operating income		25,366	[1]	36,598	[1]	29,374	[1]	41,291	[1]	34,070	[2]	31,021	[2]	26,156	[2]	21,893	[2]	132,629	113,140	75,690
Income before income taxes		19,461	[1]	30,159	[1]	17,285	[1]	33,094	[1]	29,241	[2]	27,943	[2]	22,446	[2]	16,711	[2]	99,999	96,341	61,522
Income tax provision		(6,814)	[3]	(12,265)	[3]	(6,448)	[3]	(11,590)	[3]	(5,573)	[3]	(11,196)	[3]	(9,652)	[3]	(7,353)	[3]	$ (37,117)	$ (33,774)	$ (25,811)
Net income	[3]	$ 12,647	[1]	$ 17,894	[1]	$ 10,837	[1]	$ 21,504	[1]	$ 23,668	[2]	$ 16,747	[2]	$ 12,794	[2]	$ 9,358	[2]
Basic earnings per common share		$ 0.41	[1],[3]	$ 0.58	[1],[3]	$ 0.35	[1],[3]	$ 0.70	[1],[3]	$ 0.76	[2],[3]	$ 0.54	[2],[3]	$ 0.42	[2],[3]	$ 0.30	[2],[3]	$ 2.03	$ 2.01	$ 1.10
Diluted earnings per common share		$ 0.38	[1],[3]	$ 0.55	[1],[3]	$ 0.33	[1],[3]	$ 0.64	[1],[3]	$ 0.70	[2],[3]	$ 0.50	[2],[3]	$ 0.39	[2],[3]	$ 0.28	[2],[3]	$ 1.90	$ 1.87	$ 1.06

[1] During the first, second, and fourth quarters of 2016 we incurred restructuring and reorganization charges of $(5.7) million, $5.3 million, and $1.0 million, respectively, or $(0.11), $0.10, and $0.02 per diluted share. The negative restructuring and reorganization charges in the first quarter reflect the gain on disposition of business operations (see Note 6).

[2] During the first, second, third, and fourth quarters of 2015 we incurred restructuring and reorganization charges of $0.6 million, $0.4 million, $0.8 million, and $1.3 million, respectively, or $0.01, $0.01, $0.02, and $0.03 per diluted share (see Note 6).

[3] Fluctuations in our effective income tax rate between quarters generally relates to the accounting for discrete income tax items in any given quarter, and revisions of estimates for certain income tax components during the year. •For 2016: Our effective income tax rates for the first, second, third, and fourth quarters were 35%, 37%, 41%, and 35%, respectively. •For 2015: Our effective income tax rates for the first, second, third, and fourth quarters were 44%, 43%, 40%, and 19%, respectively. The low fourth quarter rate can be mainly attributed to full year impact of the 2015 R&D tax credits, as the legislation was not passed until December 2015.