Nature of the Business and Basis of Presentation (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
	Jul. 20, 2018	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Sale of Equity
Net loss		$ (48,486)	$ (32,581)	$ 163,500
Accumulated deficit		(361,226)		$ (312,740)
Common stock
Sale of Equity
Conversion of redeemable convertible preferred stock to common stock (in shares)	51,845,438
Net loss
IPO
Sale of Equity
Shares issued (in shares)	12,055,450
Net proceeds after deducting underwriting discounts and commissions and other offering costs	$ 254,300
IPO | Common stock
Sale of Equity
Shares issued (in shares)	12,055,450
Overallotment
Sale of Equity
Shares issued (in shares)	1,572,450