Stock-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Shares Available for Grant
Beginning number of shares, available for grant (shares)	2,726,225
2018 plan annual increase (shares)	3,689,000
Granted (shares)	(6,902)
Canceled (shares)	12,406
Ending number of shares, available for grant (shares)	6,422,345	2,726,225
Shares Subject to Options Outstanding
Beginning number of shares, outstanding (shares)	4,494,889
Granted (shares)	6,902
Exercised (shares)	(242,003)
Canceled (shares)	(31,798)
Ending number of shares, outstanding (shares)	4,227,990	4,494,889
Options vested and exercisable, number of options (shares)	2,034,629
Weighted-Average Exercise Price
Beginning balance of options outstanding (in usd per share)	$ 10.90
Granted (in usd per share)	84.61
Exercised (in usd per share)	6.21
Canceled (in usd per share)	10.90
Ending balance of options outstanding (in usd per share)	11.29	$ 10.90
Options vested and exercisable, weighted average exercise price per share (in usd per share)	$ 5.77
Weighted-Average Remaining Contractual Life and Aggregate Intrinsic Value
Options outstanding, weighted average remaining contractual term (in years)	7 years 6 months	7 years 8 months 12 days
Options outstanding, aggregate intrinsic value	$ 253,161	$ 306,392
Options vested and exercisable, weighted average remaining contractual term (in years)	7 years 1 month 6 days
Options vested and exercisable, aggregate intrinsic value	$ 130,646
Restricted Stock Units
Shares Available for Grant
Granted (shares)	(14,548)
Canceled (shares)	16,164