Quarterly Financial Data (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 Data [Abstract]
Revenue	$ 124,503		$ 116,761		$ 105,318		$ 102,575		$ 100,848		$ 98,388		$ 93,586		$ 78,243		$ 449,157	$ 371,065		$ 312,828
Gross margin	50,890		46,078		42,310		43,681		39,801		37,502		37,594		32,630		182,959	147,527		131,619
Net income	$ 16,272		$ 14,272		$ 11,041		$ 12,018		$ 11,005		$ 8,968		$ 8,440		$ 7,765		$ 53,603	$ 36,178		$ 49,791
Earnings per common share:
Basic (in dollars per share)	$ 0.30	[1]	$ 0.26	[1]	$ 0.20	[1]	$ 0.22	[1]	$ 0.21	[1]	$ 0.17	[1]	$ 0.17	[1]	$ 0.16	[1]	$ 0.98	$ 0.70	[1]	$ 1.03
Diluted (in dollars per share)	$ 0.29	[1]	$ 0.26	[1]	$ 0.20	[1]	$ 0.22	[1]	$ 0.20	[1]	$ 0.16	[1]	$ 0.16	[1]	$ 0.15	[1]	$ 0.97	$ 0.67	[1]	$ 0.97

[1] The sum of the quarterly earnings per share (basic and diluted) may not agree to the earnings per share for the year due to the timing of common stock issuances.