Quarterly Financial Results (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, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Results (Unaudited) [Abstract]
Total revenues	$ 83,128	$ 87,474	$ 81,796	$ 74,698	$ 70,433	$ 70,174	$ 65,587	$ 56,245	$ 327,096	$ 262,439	$ 214,952
Total revenues	83,128	87,474	81,796	74,698	70,433	70,174	65,587	56,245	327,096	262,439	214,952
Gross margin	25,772	28,227	26,757	22,647	21,306	22,317	21,222	16,289	103,403	81,134	62,767
Income from operations	7,038	7,537	6,554	4,955	4,940	5,717	4,881	3,054	26,084	18,532	11,517
Income before income taxes	7,021	7,449	6,527	4,988	4,992	5,729	4,888	3,036	25,985	18,645	11,748
Net income	$ 4,376	$ 5,142	$ 3,603	$ 2,986	$ 2,721	$ 3,466	$ 2,767	$ 1,793	$ 16,107	$ 10,747	$ 6,480
Basic net income per share	$ 0.14	$ 0.17	$ 0.12	$ 0.10	$ 0.10	$ 0.12	$ 0.10	$ 0.07	$ 0.54	$ 0.39	$ 0.24
Diluted net income per share	$ 0.14	$ 0.16	$ 0.12	$ 0.10	$ 0.09	$ 0.12	$ 0.10	$ 0.06	$ 0.52	$ 0.37	$ 0.23