Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accrued Expenses
Research and development expenses	$ 866	$ 1,166
Employee compensation expenses	1,899	1,732
Commercial expenses	322
Vixen contract payable.	100	100
Other	323	380
Total accrued expenses	$ 3,510	$ 3,378