Net Income (Loss) Per Share Attributable to Common Stockholders (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 (loss)	$ (13,053)	$ 121	$ (27,417)	$ 5,148
Accretion of redeemable convertible preferred stock	0	(157)	(308)	(471)
Net income attributable to participating securities				(3,788)
Net income (loss) attributable to common stockholders - Basic	(13,053)	(36)	(27,725)	889
Undistributed earnings reallocated to participating securities		26		361
Net income (loss) attributable to common stockholders - Diluted	$ (13,053)	$ (10)	$ (27,725)	$ 1,250
Denominator:
Weighted-average shares outstanding - Basic	117,698,005	20,693,359	57,089,411	19,695,440
Effect of potentially dilutive securities:
Common stock options		11,433,176		10,917,099
Weighted-average shares outstanding - Diluted	117,698,005	32,126,535	57,089,411	30,612,539
Net income (loss) per share attributable to common stockholders:
Basic	$ (0.11)		$ (0.49)	$ 0.05
Diluted	$ (0.11)		$ (0.49)	$ 0.04