Accrued Expenses - Components of Accrued Expenses (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Payables And Accruals [Abstract]
Accrued clinical	$ 10,692	$ 4,536
Accrued bonus	1,250	2,178
Professional fees	761	647
Accrued vacation	433	310
Accrued payroll	410	518
Accrued severance	196
Other	2,047	1,366
Total accrued expenses	$ 15,789	$ 9,555