Stock Option Plans - Activity Under Compensation Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Shares Available for Grant
Beginning balance (in shares)	57,889	328,290
Authorized (in shares)	2,317,000	223,664
Options granted (in shares)	(848,023)	(629,716)
Options cancelled (in shares)	(27,824)	(135,651)
Ending balance (in shares)	1,554,690	57,889	328,290
Number of Shares
Beginning balance (in shares)	4,364,377	4,308,890
Options granted (in shares)	(848,023)	(629,716)
Options exercised (in shares)	(873,786)	(438,578)
Options cancelled (in shares)	(27,824)	(135,651)
Ending balance (in shares)	4,310,790	4,364,377	4,308,890
Weighted Average Exercise Price
Beginning balance (in USD per share)	$ 3.79	$ 3.09
Options granted (in USD per share)	22.77	6.51
Options exercised (in USD per share)	1.77	1.50
Options cancelled (in USD per share)	8.71	1.56
Ending balance (in USD per share)	$ 7.91	$ 3.79	$ 3.09
Weighted Average Remaining Contractual Term (in Years), awards outstanding	7 years 3 months 7 days	7 years 4 months 9 days	7 years 9 months 21 days
Aggregate Intrinsic Value, awards outstanding	$ 140,234	$ 33,132	$ 5,073
Number of Shares, vested and exercisable (in shares)	2,514,891
Weighted Average Exercise Price, vested and exercisable (in USD per share)	$ 4.27
Weighted Average Remaining Contractual Term (in Years), vested and exercisable	6 years 4 months 17 days
Aggregate Intrinsic Value, vested and exercisable	$ 90,854
Number of Shares, vested and expect to vest (in shares)	4,310,790
Weighted Average Exercise Price, vested and expected to vest (in dollars per share)	$ 7.91
Weighted Average Remaining Contractual Term (in Years), vested and expected to vest	7 years 3 months 7 days
Aggregate Intrinsic Value, vested and expected to vest	$ 140,234