Debt (Schedule of Convertible Debt) (Details) - Convertible Debt [Member] - 3.00% Exchangeable senior notes due 2024(2) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Interest Expense, Debt, Excluding Amortization	$ 25.5
Amortization of Debt Issuance Costs and Discounts	32.5	$ 31.0	$ 31.4
Debt Instrument, Face Amount	849.5	849.5		$ 849.5
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	(150.5)	(182.7)
Convertible Debt	699.0	666.8
Debt Instrument, Convertible, Carrying Amount of Equity Component	$ 220.0	$ 220.0

Amount of amortization expense attributable to debt discount (premium) and debt issuance costs.

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-Name Accounting Standards Codification
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Including the current and noncurrent portions, carrying amount of debt identified as being convertible into another form of financial instrument (typically the entity's common stock) as of the balance sheet date, which originally required full repayment more than twelve months after issuance or greater than the normal operating cycle of the company.

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The carrying amount of the equity component of convertible debt which may be settled in cash upon conversion.

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Face (par) amount of debt instrument at time of issuance.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of unamortized debt discount (premium) and debt issuance costs.

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Represents the portion of interest incurred in the period on debt arrangements that was charged against earnings, excluding amortization of debt discount (premium) and financing costs.

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