Income Taxes - Carryforwards (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards
Foreign pre-tax loss	$ 8.2	$ 3.7
Federal
Operating Loss Carryforwards
Net operating loss carryforwards	561.8
Federal | R&D credit
Operating Loss Carryforwards
Tax credit carryforwards	29.0
State
Operating Loss Carryforwards
Net operating loss carryforwards	435.9
State | R&D credit
Operating Loss Carryforwards
Tax credit carryforwards	$ 9.5