Unaudited Quarterly Financial Data - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended																12 Months Ended
		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information
Total revenues		$ 197,288		$ 186,960		$ 182,641		$ 185,631		$ 193,697		$ 185,003		$ 184,558		$ 188,028		$ 752,520	$ 751,286	$ 747,468
Total cost of revenues (exclusive of depreciation)		95,497		92,182		91,796		100,266		99,087		94,470		93,682		102,104		379,741	389,343	377,165
Operating income		34,070	[1]	31,021	[1]	26,156	[1]	21,893	[1]	19,125	[2]	13,831	[2]	21,820	[2]	20,914	[2]	113,140	75,690	76,704
Income before income taxes		29,241	[1]	27,943	[1]	22,446	[1]	16,711	[1]	16,715	[2]	10,064	[2]	17,741	[2]	17,002	[2]	96,341	61,522	61,519
Income tax provision		(5,573)	[3]	(11,196)	[3]	(9,652)	[3]	(7,353)	[3]	(5,331)	[3],[4]	(4,831)	[3],[4]	(8,338)	[3],[4]	(7,311)	[3],[4]	$ (33,774)	$ (25,811)	$ (16,251)
Net income	[3]	$ 23,668	[1]	$ 16,747	[1]	$ 12,794	[1]	$ 9,358	[1]	$ 11,384	[2],[4]	$ 5,233	[2],[4]	$ 9,403	[2],[4]	$ 9,691	[2],[4]
Basic earnings per common share		$ 0.76	[1],[3]	$ 0.54	[1],[3]	$ 0.42	[1],[3]	$ 0.30	[1],[3]	$ 0.37	[2],[3],[4]	$ 0.16	[2],[3],[4]	$ 0.29	[2],[3],[4]	$ 0.30	[2],[3],[4]	$ 2.01	$ 1.10	$ 1.41
Diluted earnings per common share		$ 0.70	[1],[3]	$ 0.50	[1],[3]	$ 0.39	[1],[3]	$ 0.28	[1],[3]	$ 0.34	[2],[3],[4]	$ 0.15	[2],[3],[4]	$ 0.28	[2],[3],[4]	$ 0.28	[2],[3],[4]	$ 1.87	$ 1.06	$ 1.38

[1] During the first, second, third, and fourth quarters of 2015 we incurred restructuring expenses 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).

[2] During the first, third, and fourth quarters of 2014 we incurred restructuring expenses of $1.2 million, $7.8 million, and $4.9 million, respectively, or $0.02, $0.12, and $0.10 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 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. •For 2014: Our effective income tax rates for the first, second, third, and fourth quarters were 43%, 47%, 48%, and 32%, respectively. The low fourth quarter rate can be mainly attributed to full year impact of the 2014 R&D tax credits, as the legislation was not passed until December 2014.

[4] As explained in Note 7, our 2014 results have been adjusted as a result of the immaterial correction made to the fourth quarter 2014 income tax provision.