Accrued Expenses - Components of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued clinical	$ 43,297	$ 23,643
Accrued bonus	3,388	2,995
Income tax payable	987
Professional fees	808	539
Accrued vacation	797	513
Accrued payroll	795	596
Accrued severance		29
Other	2,369	1,946
Total accrued expenses	$ 52,441	$ 30,261