Equity Incentive Plans - Summary of Stock Option Activity under 2015 Plan and 2018 Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options Outstanding, Beginning balance	2,339,157
Options Outstanding, Granted	773,125
Options Outstanding, Cancelled	(9,119)
Options Outstanding, Exercised	(63,014)
Options Outstanding, Ending balance	3,040,149
Options Outstanding, Exercisable	776,012
Weighted-Average Exercise Price, Beginning balance	$ 6.40
Weighted-Average Exercise Price, Granted	24.84
Weighted-Average Exercise Price, Cancelled	9.28
Weighted Average Exercise Price, Exercised	1.39
Weighted-Average Exercise Price, Ending balance	11.18
Weighted-Average Exercise Price, Exercisable	$ 5.39
Weighted-Average Remaining Term, Ending balance	8 years 9 months 18 days
Weighted-Average Remaining Term, Exercisable	8 years 2 months 12 days
Aggregate Intrinsic Value, Ending balance	$ 42,418
Aggregate Intrinsic Value, Exercisable	$ 15,224