Stock-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Shares Available for Grant
Beginning number of shares, available for grant		3,556,507	1,698,790
Shares authorized (in shares)		0	3,658,602
Shares forfeited (in shares)	(508,847)	0	(508,847)
Granted (in shares)		(324,579)	(2,088,639)
Exercised (in shares)		0	0
Canceled (in shares)		12,636	795,371
Repurchase of early exercised shares		0	1,230
Ending number of shares, available for grant		3,244,564	3,556,507	1,698,790
Shares Subject to Options Outstanding
Beginning number of shares, outstanding		7,588,405	7,391,052
Granted (in shares)		324,579	2,088,639
Exercised (in shares)		(2,999,419)	(1,007,387)
Canceled (in shares)		(418,676)	(883,899)
Ending number of shares, outstanding		4,494,889	7,588,405	7,391,052
Weighted-Average Exercise Price
Beginning balance of options outstanding (in dollars per share)		$ 4.58	$ 3.63
Granted (in dollars per share)		88.18	7.19
Exercised (in dollars per share)		3.87	3.09
Canceled (in dollars per share)		6.64	4.57
Ending balance of options outstanding (in dollars per share)		$ 10.90	$ 4.58	$ 3.63
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Options outstanding, Weighted average remaining contractual term (in years)		7 years 8 months 12 days	8 years 3 months 18 days	8 years 7 months 6 days
Options outstanding, Aggregate intrinsic value		$ 306,392	$ 250,495	$ 3,325
Options vested and exercisable, Number of options (in shares)		1,908,216
Options vested and exercisable, Weighted average exercise price per share (in dollars per share)		$ 5.27
Options vested and exercisable, Weighted average remaining contractual term (in years)		7 years 2 months 12 days
Options vested and exercisable, Aggregate intrinsic value (in dollars per share)		$ 139,337