Accrued Expenses - Summary of Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued compensation	$ 13,494	$ 20,843
Accrued research and development costs	25,036	14,777
Accrued professional fees	5,144	5,441
Accrued other	3,076	1,086
Total accrued expenses	$ 46,750	$ 42,147