Quarterly Financial Results (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, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Results (Unaudited) [Abstract]
Total revenues	$ 125,842	$ 116,971	$ 116,709	$ 97,170	$ 97,465	$ 96,758	$ 94,167	$ 84,935	$ 456,692	$ 373,325	$ 327,096
Gross margin	40,491	39,537	37,848	31,459	33,124	33,863	30,598	25,514	149,335	123,099	103,403
Income from operations	9,730	12,395	11,416	5,421	8,586	10,346	7,538	5,208	38,963	31,678	26,084
Income before income taxes	9,263	11,943	11,040	5,274	8,590	10,257	7,402	5,249	37,520	31,497	25,985
Net income	$ 6,424	$ 7,306	$ 6,387	$ 3,045	$ 5,513	$ 7,234	$ 4,562	$ 4,123	$ 23,163	$ 21,432	$ 16,107
Basic net income per share (in dollars per share)	$ 0.20	$ 0.23	$ 0.20	$ 0.10	$ 0.18	$ 0.24	$ 0.15	$ 0.14	$ 0.73	$ 0.71	$ 0.54
Diluted net income per share (in dollars per share)	$ 0.19	$ 0.22	$ 0.19	$ 0.09	$ 0.17	$ 0.23	$ 0.14	$ 0.13	$ 0.70	$ 0.67	$ 0.52