Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Expenses
Accrued Payroll and Employee Benefits	$ 4,400	$ 6,348
Accrued Research and Development Contract Costs	1,766	11,399
Accrued Professional Fees	620	1,408
Other Accrued Expenses	221	300
Accrued Expenses	$ 7,007	$ 19,455