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Investments (Aggregate Fair Value And Gross Unrealized Loss Of Fixed Maturity Securities And Equity Securities In An Unrealized Loss Position) (Details)
$ in Millions
Dec. 31, 2016
USD ($)
securities
Dec. 31, 2015
USD ($)
securities
Investment [Line Items]    
Number of Securities, Less than 12 Months | securities 3,330 4,060
Estimated Fair Value, Less than 12 Months $ 6,220.3 $ 7,327.4
Gross unrealized loss, less than 12 months $ (138.5) $ (321.5)
Number of Securities, 12 Months or Greater | securities 425 587
Estimated Fair Value, 12 Months or Greater $ 816.0 $ 840.9
Gross unrealized loss, 12 Months or greater (42.4) (126.0)
Carrying value of fixed maturity investments that did not produce income $ 0.5 $ 0.2
Fixed Maturity Securities [Member]    
Investment [Line Items]    
Number of Securities, Less than 12 Months | securities 2,878 3,268
Estimated Fair Value, Less than 12 Months $ 5,987.2 $ 7,066.3
Gross unrealized loss, less than 12 months $ (127.8) $ (290.6)
Number of Securities, 12 Months or Greater | securities 425 587
Estimated Fair Value, 12 Months or Greater $ 816.0 $ 840.9
Gross unrealized loss, 12 Months or greater $ (42.4) $ (126.0)
Fixed Maturity Securities [Member] | United States Government Securities [Member]    
Investment [Line Items]    
Number of Securities, Less than 12 Months | securities 51 48
Estimated Fair Value, Less than 12 Months $ 359.9 $ 248.4
Gross unrealized loss, less than 12 months $ (5.7) $ (1.6)
Number of Securities, 12 Months or Greater | securities 0 2
Estimated Fair Value, 12 Months or Greater $ 0.0 $ 0.9
Gross unrealized loss, 12 Months or greater $ 0.0 $ 0.0
Fixed Maturity Securities [Member] | Government Sponsored Securities [Member]    
Investment [Line Items]    
Number of Securities, Less than 12 Months | securities 18 13
Estimated Fair Value, Less than 12 Months $ 26.4 $ 18.3
Gross unrealized loss, less than 12 months $ (0.3) $ (0.1)
Number of Securities, 12 Months or Greater | securities 1 6
Estimated Fair Value, 12 Months or Greater $ 1.0 $ 8.2
Gross unrealized loss, 12 Months or greater $ (0.1) $ (0.1)
Fixed Maturity Securities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]    
Investment [Line Items]    
Number of Securities, Less than 12 Months | securities 1,022 198
Estimated Fair Value, Less than 12 Months $ 1,849.0 $ 467.8
Gross unrealized loss, less than 12 months $ (55.2) $ (4.0)
Number of Securities, 12 Months or Greater | securities 28 43
Estimated Fair Value, 12 Months or Greater $ 60.7 $ 83.0
Gross unrealized loss, 12 Months or greater $ (3.2) $ (5.2)
Fixed Maturity Securities [Member] | Corporate Securities [Member]    
Investment [Line Items]    
Number of Securities, Less than 12 Months | securities 1,272 2,492
Estimated Fair Value, Less than 12 Months $ 2,640.6 $ 4,912.3
Gross unrealized loss, less than 12 months $ (49.5) $ (267.2)
Number of Securities, 12 Months or Greater | securities 203 372
Estimated Fair Value, 12 Months or Greater $ 422.8 $ 447.0
Gross unrealized loss, 12 Months or greater $ (27.1) $ (110.5)
Fixed Maturity Securities [Member] | Residential Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Number of Securities, Less than 12 Months | securities 430 298
Estimated Fair Value, Less than 12 Months $ 905.8 $ 668.3
Gross unrealized loss, less than 12 months $ (15.3) $ (7.6)
Number of Securities, 12 Months or Greater | securities 114 119
Estimated Fair Value, 12 Months or Greater $ 136.9 $ 186.3
Gross unrealized loss, 12 Months or greater $ (4.6) $ (7.2)
Fixed Maturity Securities [Member] | Commercial Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Number of Securities, Less than 12 Months | securities 19 66
Estimated Fair Value, Less than 12 Months $ 61.2 $ 263.0
Gross unrealized loss, less than 12 months $ (0.3) $ (4.3)
Number of Securities, 12 Months or Greater | securities 24 17
Estimated Fair Value, 12 Months or Greater $ 60.8 $ 38.5
Gross unrealized loss, 12 Months or greater $ (3.4) $ (0.4)
Fixed Maturity Securities [Member] | Other Debt Securities [Member]    
Investment [Line Items]    
Number of Securities, Less than 12 Months | securities 66 153
Estimated Fair Value, Less than 12 Months $ 144.3 $ 488.2
Gross unrealized loss, less than 12 months $ (1.5) $ (5.8)
Number of Securities, 12 Months or Greater | securities 55 28
Estimated Fair Value, 12 Months or Greater $ 133.8 $ 77.0
Gross unrealized loss, 12 Months or greater $ (4.0) $ (2.6)
Equity Securities [Member]    
Investment [Line Items]    
Number of Securities, Less than 12 Months | securities 452 792
Estimated Fair Value, Less than 12 Months $ 233.1 $ 261.1
Gross unrealized loss, less than 12 months $ (10.7) $ (30.9)
Number of Securities, 12 Months or Greater | securities 0 0
Estimated Fair Value, 12 Months or Greater $ 0.0 $ 0.0
Gross unrealized loss, 12 Months or greater $ 0.0 $ 0.0