Net Loss Per Share - Calculations of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Numerator:
Net loss	$ (21,149)	$ (20,767)	$ (19,426)	$ (15,938)	$ (11,941)	$ (12,102)	$ (61,342)	$ (39,981)	$ (59,900)
Net loss attributable to common stockholders	$ (21,149)			$ (15,938)			$ (61,342)	$ (39,981)
Denominator:
Weighted average common shares outstanding, basic and diluted	25,829,105			19,619,239			25,814,593	7,332,052
Net loss per share attributable to common stockholders, basic and diluted	$ (0.82)			$ (0.81)			$ (2.38)	$ (5.45)