Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accrued Expenses
Accrued compensation	$ 1,072	$ 632
Accrued contracted research costs	1,059	1,357
Accrued professional fees	193	298
Accrued other	202	145
Accrued severance	204
Total	$ 2,730	$ 2,432