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Changes In Deferred Tax Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Tax Credit Carryforward [Line Items]
Net operating loss tax carryovers	$ 307.5	$ 336.1
Tax credits	49.7	92.6
Gross deferred income tax assets	673.6	742.9
Valuation allowance	(194.8)	(274.0)
Total deferred income tax assets	$ 478.8	468.9
Tax Cuts and Jobs Act, Transition Tax Liability
Tax Credit Carryforward [Line Items]
Net operating loss tax carryovers		355.9
Tax credits		63.2
Gross deferred income tax assets		733.3
Valuation allowance		(264.4)
Total deferred income tax assets		468.9
Tax Cuts and Jobs Act, Transition Tax Liability | Scenario, Adjustment
Tax Credit Carryforward [Line Items]
Net operating loss tax carryovers		19.8
Tax credits		(29.4)
Gross deferred income tax assets		(9.6)
Valuation allowance		9.6
Total deferred income tax assets		$ 0.0

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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