Accrued Expenses - Summary of Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued compensation	$ 5,201	$ 20,843
Accrued research and development costs	23,588	14,777
Accrued professional fees	6,774	5,441
Accrued other	1,636	1,086
Total accrued expenses	$ 37,199	$ 42,147