Income Taxes - Operating loss carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Federal
Operating loss carryforwards
Net operating loss carryforwards	$ 119.8
Federal | Research and development carryforwards
Operating loss carryforwards
Research and development tax credit carryforwards	23.4	$ 13.7
State
Operating loss carryforwards
Net operating loss carryforwards	$ 111.0