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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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