Net Income Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Numerator:
Net income, as reported	$ 7,990	$ 5,992	$ 21,125	$ 17,030
Less: net income attributable to participating securities	(133)	(101)	(346)	(277)
Net income available to common shareholders-basic and diluted	$ 7,857	$ 5,891	$ 20,779	$ 16,753
Basic:
Weighted-average shares of common stock outstanding	36,936,791	36,053,462	36,822,423	35,852,081
Less: weighted-average shares of unvested restricted common stock outstanding	(613,235)	(605,978)	(604,473)	(584,489)
Weighted-average number of common shares used in computing basic net income per common share	36,323,556	35,447,484	36,217,950	35,267,592
Net income per share applicable to common shareholders-basic	$ 0.22	$ 0.17	$ 0.57	$ 0.48
Diluted:
Weighted-average number of common shares issuable upon exercise of outstanding stock options, based on the treasury stock method	1,169,883	1,262,393	1,252,747	1,317,987
Weighted-average number of restricted stock outstanding , based on the treasury stock method	126,830	119,641	129,751	117,719
Weighted-average number of common shares used in computing diluted net income per common share	37,620,269	36,829,518	37,600,448	36,703,298
Net income per share applicable to common shareholders-diluted	$ 0.21	$ 0.16	$ 0.55	$ 0.46