Net Loss Per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (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
Earnings Per Share [Abstract]
Net loss	$ (14,191)	$ (19,454)	$ (19,726)	$ (14,060)	$ (8,310)	$ (7,885)	$ (7,429)	$ (7,056)	$ (67,431)	$ (30,680)	$ (26,014)
Accretion of convertible preferred stock to redemption value					(16)	(44)	(44)	(43)		(147)	(153)
Net loss attributable to common stockholders-basic and diluted	$ (14,191)	$ (19,454)	$ (19,726)	$ (14,060)	$ (8,326)	$ (7,929)	$ (7,473)	$ (7,099)	$ (67,431)	$ (30,827)	$ (26,167)
Weighted-average shares outstanding									26,603,512	4,486,569	1,090,731
Less: weighted average shares subject to repurchase									(21,622)	(45,906)	(213,799)
Weighted average shares used to compute basic and diluted net loss per share	28,003,957	27,861,523	25,564,249	24,849,229	14,229,775	1,329,610	1,136,259	1,075,932	26,581,890	4,440,663	876,932
Net loss per share attributable to common stockholders, basic and diluted	$ (0.51)	$ (0.70)	$ (0.77)	$ (0.57)	$ (0.59)	$ (5.96)	$ (6.58)	$ (6.60)	$ (2.54)	$ (6.94)	$ (29.84)