Net Loss per Share - Summary of Basic and Diluted Net loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 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
Numerator:
Net loss	$ (17,714)	$ (13,383)	$ (10,873)	$ (8,602)	$ (6,034)	$ (7,367)	$ (41,970)	$ (22,003)
Accruing dividends on Series A convertible preferred stock	0			(756)			(592)	(2,244)
Adjustment to accumulated deficit in connection with repurchase of Series Seed convertible preferred stock	0			0		$ (22)	0	(22)
Net loss attributable to common stockholders	$ (17,714)			$ (9,358)			$ (42,562)	$ (24,269)
Denominator:
Weighted average common shares outstanding—basic and diluted (in shares)	14,562,000			459,000			9,866,000	459,000
Net loss per share attributable to common stock holders- basic and diluted (usd per share)	$ (1.22)			$ (20.39)			$ (4.31)	$ (52.92)