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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)