Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued clinical trial costs	$ 17,332	$ 14,034
Accrued compensation	12,820	13,546
Accrued professional fees	4,003	3,706
Lease liabilities, current	3,145	0
Other accrued expenses	3,500	5,249
Total	$ 40,800	$ 36,535