Quarterly Financial Data (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
Quarterly Financial Data [Abstract]
Revenue	$ 100,848		$ 98,388		$ 93,586		$ 78,243		$ 76,702		$ 78,628		$ 78,303		$ 79,195		$ 371,065		$ 312,828		$ 258,785
Gross margin	39,801		37,502		37,594		32,630		31,300		33,843		33,025		33,451		147,527		131,619		105,820
Net income	$ 11,005		$ 8,968		$ 8,440		$ 7,765		$ 23,201		$ 9,806		$ 13,178		$ 3,606		$ 36,178		$ 49,791		$ 31,408
Earnings per common share:
Basic (in dollars per share)	$ 0.21	[1]	$ 0.17	[1]	$ 0.17	[1]	$ 0.16	[1]	$ 0.48	[1]	$ 0.20	[1]	$ 0.27	[1]	$ 0.08	[1]	$ 0.70	[1]	$ 1.03	[1]	$ 0.60
Diluted (in dollars per share)	$ 0.20	[1]	$ 0.16	[1]	$ 0.16	[1]	$ 0.15	[1]	$ 0.44	[1]	$ 0.19	[1]	$ 0.25	[1]	$ 0.07	[1]	$ 0.67	[1]	$ 0.97	[1]	$ 0.56

[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.