Net Loss per Share - Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net loss	$ (15,478)	$ (16,958)	$ (26,504)	$ (32,054)
Denominator:
Weighted average number of common shares outstanding—basic	36,009,109	25,040,131	35,245,796	25,030,291
Net loss per share attributable to common stockholders—basic and diluted	$ (0.43)	$ (0.68)	$ (0.75)	$ (1.28)