Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accrued Expenses
Accrued payroll and related expenses	$ 1,794	$ 1,174
Accrued external research and development expense	1,163	1,225
Accrued professional and consulting expense	339	382
Accrued other	64	15
Accrued expenses	$ 3,360	$ 2,796