Net Loss per Share - 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
Numerator:
Net loss	$ (19,634)	$ (14,620)	$ (12,555)	$ (7,971)	$ (8,182)	$ (4,161)	$ (2,677)	$ (1,689)	$ (54,780)	$ (16,709)	$ (6,102)
Accretion of convertible preferred stock to redemption value										(1,291)	(875)
Net loss attributable to common stockholders									$ (54,780)	$ (18,000)	$ (6,977)
Denominator:
Weighted average common shares outstanding, basic and diluted									23,532,400	6,748,037	6,394,916
Net loss per share attributable to common stockholders, basic and diluted	$ (0.50)	$ (0.38)	$ (1.45)	$ (1.15)	$ (1.24)	$ (0.68)	$ (0.45)	$ (0.29)	$ (2.33)	$ (2.67)	$ (1.09)