Share‑Based Compensation - Share-based compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 2,368	$ 231
Additional share-based compensation expense due to modifications	700
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	1,046	4
Additional share-based compensation expense due to modifications	600
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	1,322	$ 227
Additional share-based compensation expense due to modifications	$ 100