Accrued Expenses (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Accrued Expenses
Accrued research and development expenses	$ 8,854	$ 11,813
Accrued employee compensation and benefits	5,064	7,089
Accrued legal and professional fees	2,005	1,087
Other	576	426
Accrued expenses	$ 16,499	$ 20,415