Income Taxes - Changes in Deferred Tax Assets (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Tax Credit Carryforward [Line Items]
Net operating loss tax carryovers	$ 357.0	$ 336.1
Tax credits	62.4	92.6
Gross deferred income tax assets	733.6	742.9
Valuation allowance	(264.7)	(274.0)
Total deferred income tax assets	468.9	$ 468.9
Change
Tax Credit Carryforward [Line Items]
Net operating loss tax carryovers	20.9
Tax credits	(30.2)
Gross deferred income tax assets	(9.3)
Valuation allowance	9.3
Total deferred income tax assets	$ 0.0