Organization, Liquidity and Management's Plan, and Summary of Significant Accounting Policies - Schedule of Computation of Basic and Diluted Net Loss per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss attributable to common stockholders	$ (7,674)	$ (6,672)	$ (12,598)	$ (10,223)
Denominator:
Weighted-average common shares outstanding	72,104,334	52,973,371	72,021,742	48,955,952
Less: Weighted-average shares subject to repurchase	(183,095)	(206,260)	(183,095)	(225,571)
Denominator for basic and diluted net loss per share	71,921,239	52,767,111	71,838,647	48,730,381
Net loss per share:
Basic and diluted	$ (0.11)	$ (0.13)	$ (0.18)	$ (0.21)