Investment Income and Gains and Losses (Components of Net Investment Income) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 346.0	$ 307.7	$ 321.7
Less: investment expenses	(13.9)	(11.4)	(11.0)
Net investment income	332.1	296.3	310.7
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	283.2	239.3	216.9
Limited Partnerships Interest [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	30.0	35.7	68.2
Mortgage Loans on Real Estate [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	15.8	16.1	18.0
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	6.4	12.0	15.6
Other Investments [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 10.6	$ 4.6	$ 3.0

Amount before accretion (amortization) of purchase discount (premium) of interest income and dividend income on nonoperating securities.

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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