Accrued Expenses & Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued expenses and other current liabilities:
Accrued compensation and benefits	$ 7,886	$ 4,111
Accrued consulting and professional fees	3,841	1,142
Accrued research and development	1,855	5,998
Accrued other	5,357	2,510
Accrued expenses and other current liabilities	$ 18,939	$ 13,761