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Fair Value (Fair Value of Financial Instruments) (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Carrying Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash and cash equivalents $ 227.2 $ 282.6
Total financial assets 9,261.3 8,696.1
Debt 786.8 786.4
Carrying Value [Member] | Other Investments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of investments 577.1 497.8
Carrying Value [Member] | Fixed Maturities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of investments 7,890.5 7,331.3
Carrying Value [Member] | Equity Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of investments 566.5 584.4
Estimated Fair Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash and cash equivalents 227.2 282.6
Total financial assets 9,270.2 8,695.9
Debt 873.3 841.9
Estimated Fair Value [Member] | Other Investments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of investments 586.0 497.6
Estimated Fair Value [Member] | Fixed Maturities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of investments 7,890.5 7,331.3
Estimated Fair Value [Member] | Equity Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of investments $ 566.5 $ 584.4