Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Components of accrued expenses and other current liabilities:
Research and development related costs	$ 2,308	$ 8,716
Payroll and related costs	4,426	2,345
Accounting, tax and audit services	231	484
Other	1,159	1,781
Total other current liabilities	$ 8,124	$ 13,326