Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Accrued contract research and manufacturing costs	$ 8,843	$ 12,338
Accrued compensation	8,743	9,414
Accrued professional fees	2,429	3,979
Other	513	1,135
Total accrued expenses	$ 20,528	$ 26,866