Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Expenses
Accrued Payroll and Employee Benefits	$ 6,348	$ 7,132
Accrued Research and Development Contract Costs	11,399	17,742
Accrued Professional Fees	1,408	1,146
Other Accrued Expenses	300	2,637
Accrued Expenses	$ 19,455	$ 28,657