Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Accrued Expenses
Accrued external research and development expense	$ 4,125	$ 4,088
Accrued payroll and related expenses	2,204	4,380
Accrued professional and consulting expense	920	929
Accrued other	127	213
Accrued expenses	$ 7,376	$ 9,610