Net Income Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Numerator:
Net income, as reported	$ 8,616	$ 5,720
Less: net income attributable to participating securities	(138)	(87)
Net income available to common shareholders-basic	8,478	5,633
Add-back: undistributed earnings allocated to unvested shareholders	103	87
Less: undistributed earnings reallocated to unvested shareholders	(100)	(84)
Net income available to common shareholders - diluted	$ 8,481	$ 5,636
Basic:
Weighted-average shares of common stock outstanding	37,717,140	36,839,767
Less: weighted-average shares of unvested restricted common stock outstanding	(614,633)	(557,339)
Weighted-average number of common shares used in computing basic net income per common share	37,102,507	36,282,428
Net income per share applicable to common shareholders-basic	$ 0.23	$ 0.16
Diluted:
Weighted-average number of common shares issuable upon exercise of outstanding stock options, based on the treasury stock method	1,382,015	1,324,619
Weighted-average number of common shares used in computing diluted net income per common share	38,484,522	37,607,047
Net income per share applicable to common shareholders-diluted	$ 0.22	$ 0.15