Equity Incentive Plan - Schedule of Stock Option Activity Under 2019 Plan (Detail) - 2019 Plan [Member] - USD ($) $ / shares in Units, $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2021	Dec. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options outstanding, beginning balance		4,260,753
Options granted		1,277,771
Exercised		(4,676)
Options forfeited		(350,928)
Options expired		(56,517)
Options outstanding, ending balance		5,126,403	4,260,753
Options Vested and Exercisable, September 30, 2021		1,352,295
Options Vested and Expected to Vest, September 30, 2021		5,126,403
Weighted average exercise price, beginning balance		$ 8.46
Weighted average exercise price, granted		8.03
Exercised		8.00
Weighted average exercise price, forfeited		8.27
Weighted average exercise price, expired		8.12
Weighted average exercise price, ending balance		8.37	$ 8.46
Options Vested and Exercisable, September 30, 2021		8.46
Options Vested and Expected to Vest, September 30, 2021		$ 8.37
Weighted average remaining contractual term, balance		8 years 7 months 6 days	9 years
Options Vested and Exercisable, September 30, 2021		8 years 3 months 18 days
Options Vested and Expected to Vest, September 30, 2021		8 years 7 months 6 days
Aggregate intrinsic value, beginning balance	[1]	$ 5	$ 54
Options Vested and Expected to Vest, September 30, 2021	[1]	$ 5

[1] Aggregate intrinsic value represents the difference between the estimated fair value of the underlying Common Stock and the exercise price of outstanding in-the-money options.