Unaudited Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information
Total revenues	$ 198,007	$ 190,001	$ 183,851	$ 185,007	$ 187,574	$ 182,753	$ 181,312	$ 183,092	$ 756,866	$ 734,731	$ 549,379
Total cost of revenues (exclusive of depreciation)	100,916	100,411	92,520	89,969	91,022	92,988	90,876	90,764	383,816	365,650	289,804
Operating income	22,149	21,728	23,745	28,952	27,043	22,767	22,371	24,104	96,574	96,285	74,342
Income before income taxes	17,656	17,114	18,842	23,612	22,140	19,771	15,816	18,245	77,224	75,972	33,673
Income tax provision	(1,866)	(7,701)	(6,972)	(11,806)	(10,846)	(9,292)	(6,801)	(6,751)	(28,345)	(33,690)	(11,244)
Net income	$ 15,790	$ 9,413	$ 11,870	$ 11,806	$ 11,294	$ 10,479	$ 9,015	$ 11,494	$ 48,879	$ 42,282	$ 22,429
Basic earnings per common share	$ 0.49	$ 0.29	$ 0.37	$ 0.36	$ 0.35	$ 0.32	$ 0.27	$ 0.35	$ 1.52	$ 1.29	$ 0.68
Diluted earnings per common share	$ 0.48	$ 0.29	$ 0.37	$ 0.36	$ 0.35	$ 0.32	$ 0.27	$ 0.35	$ 1.51	$ 1.28	$ 0.67