Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued Expenses
Accrued research and development expenses	$ 16,945	$ 11,813
Accrued employee compensation and benefits	7,704	7,089
Accrued legal and professional fees	3,988	1,087
Other	562	426
Accrued expenses	$ 29,199	$ 20,415