Nature of the Business and Basis of Presentation (Details) - USD ($) $ / shares in Units, $ in Thousands					3 Months Ended				6 Months Ended
	Jul. 24, 2020	May 18, 2020	Mar. 01, 2019	Feb. 04, 2019	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Liquidity
Accumulated deficit					$ 419,980				$ 419,980		$ 356,092
Net losses					37,469	$ 26,419	$ 37,191	$ 65,821	63,888	$ 103,012
Cash used in operations									61,020	104,932
Cash, cash equivalents and short-term investments					$ 252,398				252,398
Follow-On Public Offering and Private Placement Inclusive of Over-Allotment
Nature of the Business and Basis of Presentation
Aggregate proceeds from offerings net of underwriter discounts and commissions, placement agent fees and other offering costs	$ 145,925	$ 74,495	$ 82,988
Follow-On Public Offering and Private Placement | Common Class A and Class A1 common shares
Nature of the Business and Basis of Presentation
Gross proceeds from offering	$ 155,000	$ 79,570		$ 85,000
Follow-On Offering | Class A common shares
Nature of the Business and Basis of Presentation
Issuance of stock (in shares)	5,952,381	2,760,000		2,654,984
Share price (in dollars per share)	$ 21.00	$ 18.25		$ 18.26
Gross proceeds from offering									$ 47,600	$ 48,595
Private Placement | Class A1 common shares
Nature of the Business and Basis of Presentation
Issuance of stock (in shares)	1,428,572	1,600,000		2,000,000
Share price (in dollars per share)	$ 21.00	$ 18.25		$ 18.26
Over-allotment | Class A common shares
Nature of the Business and Basis of Presentation
Issuance of stock (in shares)			161,126
Share price (in dollars per share)			$ 18.26
Gross proceeds from offering			$ 2,942