Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued Expenses
Accrued external research and development expense	$ 4,088	$ 3,284
Accrued payroll and related expenses	4,380	2,826
Accrued professional and consulting expense	929	890
Accrued other	213	157
Accrued expenses	$ 9,610	$ 7,157