Equity Incentive Plans - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Subject to Options Outstanding, as of December 31, 2018	5,107,329
Shares Subject to Options Outstanding, Options granted	2,390,144
Shares Subject to Options Outstanding, Option exercised	(27,500)
Shares Subject to Options Outstanding, Outstanding as of March 31, 2019	7,469,973	5,107,329
Shares Subject to Options Outstanding, Options vested and exercisable as of March 31, 2019	160,682
Weighted-Average Exercise Price, Outstanding as of December 31, 2018	$ 7.51
Weighted-Average Exercise Price, Options granted	21.74
Weighted-Average Exercise Price, Option exercised	2.61
Weighted-Average Exercise Price, Outstanding as of March 31, 2019	12.08	$ 7.51
Weighted-Average Exercise Price, Options vested and exercisable as of March 31, 2019	$ 3.20
Weighted-Average Remaining Contractual Life (Years), Outstanding as of December 31, 2018	9 years 7 months 6 days	9 years 8 months 12 days
Weighted-Average Remaining Contractual Life (Years), Options vested and exercisable as of March 31, 2019	8 years 10 months 24 days
Aggregate Intrinsic Value, Outstanding as of December 31, 2018	$ 72,708	$ 16,343
Aggregate Intrinsic Value, Options vested and exercisable as of March 31, 2019	$ 2,968