Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued Expenses
Accrued payroll and employee benefits	$ 6,113	$ 4,575
Accrued research and development contract costs	1,590	1,186
Accrued professional fees	421	400
Other accrued expenses	335	338
Accrued expenses	$ 8,459	$ 6,499