Net Income (Loss) Per Share Attributable to Common Stockholders and Pro Forma Net Loss Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Numerator:
Net income (loss)	$ (8,724)	$ 2,036	$ (14,364)	$ 5,027
Accretion of redeemable convertible preferred stock	(154)	(158)	(308)	(313)
Net income attributable to participating securities		(1,520)		(3,835)
Net income attributable to common stockholders - Basic	(8,878)	358	(14,672)	878
Undistributed earnings reallocated to participating securities		133		331
Net income (loss) attributable to common stockholders - Diluted	$ (8,878)	$ 491	$ (14,672)	$ 1,209
Denominator:
Weighted-average shares outstanding - Basic	27,788,547	19,668,851	26,452,100	19,188,210
Effect of potentially dilutive securities:
Common stock options		9,915,374		9,745,364
Weighted-average shares outstanding - Diluted	27,788,547	29,584,225	26,452,100	28,933,574
Net income (loss) per share attributable to common stockholders:
Basic	$ (0.32)	$ 0.02	$ (0.55)	$ 0.05
Diluted	$ (0.32)	$ 0.02	$ (0.55)	$ 0.04