Pension Plans (Schedule of Fair Values of Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 7,985.3	$ 7,481.8
Equity securities	130.9	241.9
Investments Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	286.5	284.7
Equity securities	35.6	37.6
Total investment assets at fair value	$ 322.1	$ 322.3