Accrued Expenses (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Accrued Expenses
Accrued payroll and related expenses	$ 3,580	$ 4,380
Accrued external research and development expense	3,277	4,088
Accrued professional and consulting expense	1,233	929
Accrued other	143	213
Accrued expenses	$ 8,233	$ 9,610