Quarterly Financial Information (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	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, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 60,633	$ 50,808	$ 51,476	$ 51,404	$ 50,847	$ 50,293	$ 51,004	$ 52,566	$ 54,023	$ 53,804	$ 47,716	$ 43,827	$ 214,321	$ 204,710	$ 199,370
Gross Profit	43,096	40,533	41,512	41,792	40,761	40,157	40,646	42,675	43,576	44,304	38,559	34,798
Operating income	21,068	21,738	17,461	19,405	17,806	18,601	19,294	19,305	20,260	20,708	17,711	18,329	79,672	75,006	77,008
Net income	$ 16,547	$ 18,015	$ 13,579	$ 15,417	$ 15,245	$ 13,143	$ 13,542	$ 17,344	$ 18,745	$ 18,072	$ 18,067	$ 15,685	$ 63,558	$ 59,274	$ 70,569
Basic (in dollars per share)	$ 0.45	$ 0.47	$ 0.35	$ 0.40	$ 0.40	$ 0.35	$ 0.36	$ 0.47	$ 0.50	$ 0.49	$ 0.49	$ 0.43	$ 1.68	$ 1.58	$ 1.91
Diluted (in dollars per share)	$ 0.45	$ 0.47	$ 0.35	$ 0.40	$ 0.40	$ 0.34	$ 0.35	$ 0.45	$ 0.48	$ 0.46	$ 0.47	$ 0.40	$ 1.67	$ 1.53	$ 1.80