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Debt (Schedule of Convertible Debt) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net $ 132.1  
Convertible Debt [Member] | 3.00% Exchangeable senior notes due 2024    
Debt Instrument [Line Items]    
Debt Instrument, Face Amount 849.5 $ 849.5
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net (213.8) (245.2)
Convertible Debt 635.7 604.3
Debt Instrument, Convertible, Carrying Amount of Equity Component $ 220.0 $ 220.0