Net Loss per Share - Computation of Basic and Diluted Net Loss per Share of Common Stock (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share Basic [Line Items]
Net loss	$ (21,743)	$ (6,352)	$ (32,003)	$ (11,710)
Accretion of redeemable convertible preferred stock	(6)	(12)	(18)	(24)
Net loss attributable to common stockholders	$ (21,749)	$ (6,364)	$ (32,021)	$ (11,734)
Basic shares:
Weighted-average shares used to compute basic net loss per share	45,760	21,568	33,817	21,213
Diluted shares:
Weighted-average shares used to compute diluted net loss per share	45,760	21,568	33,817	21,213
Net loss per share attributable to common stockholders:
Net loss per share attributable to common stockholders, basic and diluted	$ (0.48)	$ (0.30)	$ (0.95)	$ (0.55)