Accrued Expenses (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Accrued Expenses
Payroll and related costs	$ 2,309	$ 3,108
Clinical and nonclinical trial expenses	6,163	3,495
Professional and consulting fees	4,220	1,317
Other	29	80
Total accrued expenses	12,721	8,000
Merger-related costs	$ 3,800	$ 700