Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accrued expenses
Accrued development milestones payable	$ 13,500	$ 0
Accrued employee compensation and benefits	4,219	108
Accrued clinical trial costs	10,126	6,753
Accrued professional fees	6,625	1,636
Other	1,174	285
Total accrued expenses	$ 35,644	$ 8,782