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Income From Investments - Net Realized Gains on Sales of Investments (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Net Investment Income [Line Items]                      
Realized Investment Gains                 $ 56.9 $ 38.0 $ 55.3
Realized Investment Losses                 (1.0) (5.1) (2.9)
Net Gains (Losses) on Trading Securities                 0.6 0.2 (0.3)
Net Realized Gains (Losses) on Sales of Investments $ 11.5 $ 8.1 $ 26.4 $ 10.5 $ 9.1 $ 11.6 $ 5.6 $ 6.8 56.5 33.1 52.1
Fixed Maturities [Member]                      
Net Investment Income [Line Items]                      
Gains on Sales                 8.4 17.0 16.1
Losses on Sales                 (0.9) (4.6) (1.1)
Equity Securities [Member]                      
Net Investment Income [Line Items]                      
Gains on Sales                 42.0 19.9 39.2
Losses on Sales                 0.0 (0.3) (1.6)
Real Estate Investment [Member]                      
Net Investment Income [Line Items]                      
Realized Investment Gains                 6.4 1.1 0.0
Realized Investment Losses                 0.0 0.0 (0.2)
Other Investment [Member]                      
Net Investment Income [Line Items]                      
Realized Investment Gains                 0.1 0.0 0.0
Realized Investment Losses                 $ (0.1) $ (0.2) $ 0.0