Equity Incentive Plans - Summary of Stock Option Activity (Details) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares
Shares
Shares Subject to Options Outstanding, Outstanding as of beginning of period (in shares) | shares	9,401,082
Shares Subject to Options Outstanding, Options granted (in shares) | shares	2,923,501
Shares Subject to Options Outstanding, Option exercised (in shares) | shares	(280,895)
Shares Subject to Options Outstanding, Options forfeited/cancelled (in shares) | shares	(2,503,356)
Shares Subject to Options Outstanding, Outstanding as of end of period (in shares) | shares	9,540,332	9,401,082
Shares Subject to Options Outstanding, Options vested and exercisable as of end of period (in shares) | shares	4,521,056
Weighted- Average Exercise Price
Weighted-Average Exercise Price, Outstanding as of beginning of period (in USD per share) | $ / shares	$ 13.42
Weighted-Average Exercise Price, Options granted (in USD per share) | $ / shares	9.67
Weighted-Average Exercise Price, Option exercised (in USD per share) | $ / shares	5.85
Weighted-Average Exercise Price, Options forfeited/cancelled (in USD per share) | $ / shares	14.01
Weighted-Average Exercise Price, Outstanding as of end of period (in USD per share) | $ / shares	12.33	$ 13.42
Weighted-Average Exercise Price, Options vested and exercisable as of end of period (in USD per share) | $ / shares	$ 12.74
Weighted-Average Remaining Contractual Life (in years)	7 years 6 months	8 years 1 month 6 days
Weighted-Average Remaining Contractual Life (in years), Options vested and exercisable as of end of period	6 years 7 months 6 days
Aggregate Intrinsic Value | $	$ 22,958	$ 10,182
Aggregate Intrinsic Value, Options vested and exercisable as of end of period | $	$ 12,414