Equity Incentive Plans - Summary of Stock Option Activity (Details) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Subject to Options Outstanding, as of December 31, 2018 | shares	5,107,329
Shares Subject to Options Outstanding, Options granted | shares	3,441,901
Shares Subject to Options Outstanding, Option exercised | shares	(173,712)
Shares Subject to Options Outstanding, Options forfeited/cancelled | shares	(275,657)
Shares Subject to Options Outstanding, Outstanding as of September 30, 2019 | shares	8,099,861	5,107,329
Shares Subject to Options Outstanding, Options vested and exercisable as of September 30, 2019 | shares	644,722
Weighted-Average Exercise Price, Outstanding as of December 31, 2018 | $ / shares	$ 7.51
Weighted-Average Exercise Price, Options granted | $ / shares	20.79
Weighted-Average Exercise Price, Option exercised | $ / shares	3.04
Weighted-Average Exercise Price, Options forfeited/cancelled | $ / shares	12.30
Weighted-Average Exercise Price, Outstanding as of September 30, 2019 | $ / shares	13.09	$ 7.51
Weighted-Average Exercise Price, Options vested and exercisable as of September 30, 2019 | $ / shares	$ 3.88
Weighted-Average Remaining Contractual Life (Years), Outstanding as of December 31, 2018	9 years 2 months 12 days	9 years 8 months 12 days
Weighted-Average Remaining Contractual Life (Years), Options vested and exercisable as of September 30, 2019	8 years 8 months 12 days
Aggregate Intrinsic Value, Outstanding as of December 31, 2018 | $	$ 44,015	$ 16,343
Aggregate Intrinsic Value, Options vested and exercisable as of September 30, 2019 | $	$ 8,323