Selected Quarterly Financial Data (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, 2013	Dec. 31, 2012	Dec. 31, 2011
Selected Quarterly Financial Data (Unaudited) [Abstract]
Revenue	$ 84,953	$ 81,892	$ 77,012	$ 73,854	$ 71,520	$ 69,634	$ 63,287	$ 51,717	$ 317,711	$ 256,158	$ 136,920
Gross Profit	46,970	43,697	42,092	39,145	41,971	40,093	35,727	30,901	171,904	148,692	79,689
Operating Income	25,177	22,188	22,251	18,589	19,715	22,603	18,776	7,662	88,204	68,756	33,266
Consolidated Net Income [Abstract]
Amount	$ 16,678	$ 12,205	$ 12,637	$ 10,842	$ 8,811	$ 13,753	$ 9,706	$ 3,108	$ 52,362	$ 35,378	$ 43,013
Per Share Basic (in dollars per share)	$ 0.17	$ 0.13	$ 0.13	$ 0.11	$ 0.09	$ 0.14	$ 0.10	$ 0.03	$ 0.54	$ 0.37	$ 0.45
Per Share Diluted (in dollars per share)	$ 0.14	$ 0.11	$ 0.11	$ 0.10	$ 0.08	$ 0.12	$ 0.09	$ 0.03	$ 0.46	$ 0.32	$ 0.41