Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Payables and Accruals [Abstract]
Research costs—Nordic	$ 1,228	$ 2,898
Research costs—other	8,404	5,178
Payroll and employee benefits	9,338	3,330
Professional fees	7,532	3,546
Other current liabilities	95	0
Total accrued expenses and other current liabilities	$ 26,597	$ 14,952