Stock-Based Awards - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Number of Options
Number of options, beginning balance (in shares)	11,640,734
Number of options, granted (in shares)	2,998,584
Number of options, exercised (in shares)	(259,569)
Number of options, forfeited (in shares)	(130,071)
Number of options, ending balance (in shares)	14,249,678	11,640,734
Number of options, vested and expected to vest (in shares)	12,504,946
Number of options, exercisable (in shares)	4,729,113
Weighted- Average Exercise Price
Weighted average exercise price, beginning balance (usd per share)	$ 12.96
Weighted average exercise price, granted (usd per share)	24.63
Weighted average exercise price, exercised (usd per share)	7.57
Weighted average exercise price, forfeited (usd per share)	18.02
Weighted average exercise price, ending balance (usd per share)	15.47	$ 12.96
Weighted average exercise price, vested and expected to vest (usd per share)	17.53
Weighted average exercise price, exercisable (usd per share)	$ 12.63
Stock Option Activity, Additional Disclosures
Weighted average remaining contractual life	8 years 10 days	7 years 9 months 14 days
Weighted average remaining contractual life, vested and expected to vest	8 years 4 months 24 days
Weighted average remaining contractual life, exercisable	7 years 5 months 1 day
Aggregate intrinsic value, ending balance	$ 64,492
Aggregate intrinsic value, vested and expected to vest	35,128
Aggregate intrinsic value	$ 29,163

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