Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accrued Expenses
Employee compensation expenses	$ 2,665	$ 4,948
Research and development expenses	2,045	1,437
Professional fees	276	1,123
Other	1,080	582
Total accrued expenses	$ 6,066	$ 8,090