Quarterly Financial Information (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	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, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 50,847	$ 50,293	$ 51,004	$ 52,566	$ 54,023	$ 53,804	$ 47,716	$ 43,827	$ 44,050	$ 42,602	$ 42,267	$ 40,050	$ 204,710	$ 199,370	$ 168,969
Gross Profit	40,761	40,157	40,646	42,675	43,576	44,304	38,559	34,798	35,389	33,895	33,353	32,743
Operating income	17,806	18,601	19,294	19,305	20,260	20,708	17,711	18,329	16,556	17,954	18,605	15,634	75,006	77,008	68,748
Net income	$ 15,245	$ 13,143	$ 13,542	$ 17,344	$ 18,745	$ 18,072	$ 18,067	$ 15,685	$ 17,330	$ 16,536	$ 22,348	$ 15,164	$ 59,274	$ 70,569	$ 71,378
Basic (in dollars per share)	$ 0.40	$ 0.35	$ 0.36	$ 0.47	$ 0.50	$ 0.49	$ 0.49	$ 0.43	$ 0.48	$ 0.44	$ 0.57	$ 0.40	$ 1.58	$ 1.91	$ 1.89
Diluted (in dollars per share)	$ 0.40	$ 0.34	$ 0.35	$ 0.45	$ 0.48	$ 0.46	$ 0.47	$ 0.40	$ 0.44	$ 0.41	$ 0.53	$ 0.37	$ 1.53	$ 1.80	$ 1.75