Quarterly Financial Information (unaudited) (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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 70,204	$ 66,813	$ 64,712	$ 63,753	$ 60,633	$ 50,808	$ 51,476	$ 51,404	$ 50,847	$ 50,293	$ 51,004	$ 52,566	$ 265,482	$ 214,321	$ 204,710
Gross Profit	53,760	49,004	46,013	44,268	43,096	40,533	41,512	41,792	40,761	40,157	40,646	42,675
Operating income	25,824	21,968	20,423	20,499	21,068	21,738	17,461	19,405	17,806	18,601	19,294	19,305	88,714	79,672	75,006
Net income	$ 21,929	$ 20,232	$ 19,036	$ 18,336	$ 16,547	$ 18,015	$ 13,579	$ 15,417	$ 15,245	$ 13,143	$ 13,542	$ 17,344	$ 79,533	$ 63,558	$ 59,274
Basic (in dollars per share)	$ 0.65	$ 0.59	$ 0.54	$ 0.51	$ 0.45	$ 0.47	$ 0.35	$ 0.40	$ 0.40	$ 0.35	$ 0.36	$ 0.47	$ 2.29	$ 1.68	$ 1.58
Diluted (in dollars per share)	$ 0.65	$ 0.59	$ 0.54	$ 0.51	$ 0.45	$ 0.47	$ 0.35	$ 0.40	$ 0.40	$ 0.34	$ 0.35	$ 0.45	$ 2.28	$ 1.67	$ 1.53