Significant Accounting Policies - Schedule of Summary of Calculation of Basic and Diluted Net Loss Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator:
Net loss	$ (20,892)	$ (17,358)	$ (35,979)	$ (20,499)
Series A preferred stock dividends		(532)		(1,089)
Net loss attributable to common stockholders	$ (20,892)	$ (17,890)	$ (35,979)	$ (21,588)
Denominator:
Weighted-average common shares outstanding	44,826,366	10,250,288	44,721,065	7,923,818
Less: weighted-average unvested common shares subject to repurchase	(1,577,017)	(2,360,259)	(1,861,357)	(1,186,649)
Weighted-average shares used to compute net loss per share attributable to common stockholders, basic and diluted	43,249,349	7,890,029	42,859,708	6,737,169
Net loss per common share attributable to common stockholders, basic and diluted	$ (0.48)	$ (2.27)	$ (0.84)	$ (3.20)