Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued Expenses
Accrued research and development expenses	$ 11,813	$ 9,656
Accrued employee compensation and benefits	7,089	5,678
Accrued legal and professional fees	1,087	994
Other	426	90
Accrued expenses	$ 20,415	$ 16,418