Accrued Expenses - Components of Accrued Expenses (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued clinical expenses	$ 22,576	$ 23,643
Accrued bonus	1,750	2,995
Accrued professional fees	761	539
Accrued vacation	681	513
Accrued payroll	646	596
Accrued other	2,437	1,975
Total accrued expenses	$ 28,851	$ 30,261