Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Accrued Expenses
Accrued research and development expenses	$ 8,479	$ 11,813
Accrued employee compensation and benefits	3,456	7,089
Accrued legal and professional fees	726	1,087
Other	718	426
Accrued expenses	$ 13,379	$ 20,415