Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued Liabilities, Current [Abstract]
Accrued compensation costs	$ 2,635	$ 3,364
Accrued retention payments	795	0
Accrued restructuring costs	981	0
Accrued professional service fees	904	1,262
Accrued external research and manufacturing costs	1,717	3,001
Accrued additions of property and equipment	0	243
Other accrued expenses	432	552
Accrued expenses	$ 7,464	$ 8,422