Share‑Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Number of shares
Outstanding at January 1, 2020	2,577,719
Granted	3,470,051
Forfeited	(134,331)
Outstanding at March 31, 2020	5,913,439		2,577,719
Exercisable at March 31, 2020	275,891
Vested or expected to vest at March 31, 2020	5,913,439
Weighted average exercise price per share
Outstanding at January 1, 2020	$ 5.90
Granted	13.14
Forfeited	1.02
Outstanding at March 31, 2020	9.94		$ 5.90
Exercisable at March 31, 2020	5.01
Vested or expected to vest at March 31, 2020	$ 9.94
Weighted average remaining contractual term (years)
Outstanding	9 years 6 months		9 years 3 months 18 days
Exercisable at March 31, 2020	9 years 3 months 18 days
Vested or expected to vest at March 31, 2020	9 years 6 months
Weighted average grant date fair value of options granted	$ 9.93	$ 0.73
Unrecognized compensation expense related to unvested stock option awards	$ 41.8
Weighted average period for recognition	2 years 6 months 11 days