Summary of Significant Accounting Policies - Summary of Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Consolidated net loss	$ (16,493)	$ (11,329)	$ (11,080)	$ (9,071)	$ (6,038)	$ (6,631)	$ (5,588)	$ (6,093)	$ (47,973)	$ (24,350)	$ (20,014)
Accretion to redemption value of redeemable convertible preferred stock									(15)	(416)	(1,637)
Net loss attributable to common stockholders									$ (47,988)	$ (24,766)	$ (21,651)
Denominator:
Weighted average common shares outstanding									15,900,167	895,678	839,348
Weighted average common shares subject to repurchase									(61,814)	(61,457)	(76,587)
Weighted average common shares outstanding - basic and diluted									15,838,353	834,221	762,761
Net loss per share - basic and diluted	$ (0.70)	$ (0.48)	$ (0.74)	$ (9.39)	$ (7.04)	$ (8.02)	$ (6.85)	$ (7.87)	$ (3.03)	$ (29.69)	$ (28.39)