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Investments (Unrealized Gains and Losses on Available-For-Sale and Other Securities) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Schedule of Available-for-sale Securities [Line Items] |  |  |
Balance at beginning year | $ 2,841.8 | $ 2,465.6 |
Provision for deferred income taxes | 77.4 | 17.3 | $ 49.0
Balance at end of year | 3,571.5 | 2,841.8 | 2,465.6
Net Unrealized Appreciation (Depreciation) on Investments [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Balance at beginning year | (403.5) | (462.8) | (641.4)
Net appreciation on available-for-sale fixed maturities | 364.6 | 76.6 | 227.6
Provision for deferred income taxes | (77.4) | (17.3) | (49.0)
Total adjustment | 287.2 | 59.3 | 178.6
Balance at end of year | $ (116.3) | $ (403.5) | $ (462.8)

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