Quarterly Financial Information (unaudited) (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, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 54,023	$ 53,804	$ 47,716	$ 43,827	$ 44,050	$ 42,602	$ 42,267	$ 40,050	$ 35,097	$ 33,281	$ 32,207	$ 31,603	$ 199,370	$ 168,969	$ 132,188
Gross Profit	43,576	44,304	38,559	34,798	35,389	33,895	33,353	32,743	27,406	25,863	24,780	24,540
Operating income	20,260	20,708	17,711	18,329	16,556	17,954	18,605	15,634	13,658	13,082	13,008	12,759	77,008	68,748	52,507
Net income	$ 18,745	$ 18,072	$ 18,067	$ 15,685	$ 17,330	$ 16,536	$ 22,348	$ 15,164	$ 15,944	$ 16,681	$ 14,010	$ 12,384	$ 70,569	$ 71,378	$ 59,019
Basic (in dollars per share)	$ 0.50	$ 0.49	$ 0.49	$ 0.43	$ 0.48	$ 0.44	$ 0.57	$ 0.40	$ 0.45	$ 0.48	$ 0.40	$ 0.36	$ 1.91	$ 1.89	$ 1.69
Diluted (in dollars per share)	$ 0.48	$ 0.46	$ 0.47	$ 0.40	$ 0.44	$ 0.41	$ 0.53	$ 0.37	$ 0.42	$ 0.43	$ 0.36	$ 0.32	$ 1.80	$ 1.75	$ 1.51