Pension Plans (Schedule of Fair Values of Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 7,454.4	$ 6,687.1
Equity securities	598.5	575.7
Investments Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	414.4	394.3
Equity securities	52.7	51.7
Total investment assets at fair value	$ 467.1	$ 446.0

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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