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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator--basic and diluted:
Net loss	$ (19,104)	$ (15,088)	$ (13,639)	$ (12,971)	$ (10,815)	$ (14,969)	$ (8,358)	$ (10,330)	$ (60,802)	$ (44,472)	$ (24,119)
Accretion of convertible preferred stock dividends										(2,598)	(10,011)
Accretion of convertible preferred stock to redemption value											(69,715)
Net loss attributable to common stockholders--basic and diluted									$ (60,802)	$ (47,070)	$ (103,845)
Net loss per share—basic and diluted	$ (0.80)	$ (0.68)	$ (0.70)	$ (0.71)	$ (0.59)	$ (0.84)	$ (0.47)	$ (2.85)	$ (2.90)	$ (3.22)	$ (1,519.27)
Denominator—basic and diluted:
Weighted-average common shares used to compute net loss per share—basic and diluted	23,943,241	22,239,996	19,497,581	18,231,602	18,193,027	17,899,481	17,769,514	4,541,536	20,997,211	14,619,716	68,352