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Investment Income and Gains and Losses (Components of Net Investment Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 469.7	$ 387.8	$ 346.0
Less: investment expenses	(15.3)	(15.2)	(13.9)
Net investment income	454.4	372.6	332.1
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	401.4	324.4	283.2
Limited Partnerships Interest [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	27.1	20.6	30.0
Mortgage Loans on Real Estate [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	11.2	14.4	15.8
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	3.6	3.2	6.4
Short-term and Other Investments [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 26.4	$ 25.2	$ 10.6

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

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