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Investments - Allowance for Credit Losses (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]    
Balance, Beginning of Year $ 10.7 $ 8.2
Additions for Securities for which No Previous Expected Credit Losses were Recognized 2.1 2.0
Reductions Due to Sales (0.4) (0.8)
Net (Decrease) Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized 4.5 0.1
Write-offs Charged Against Allowance (1.2)  
Balance, End of Period 15.7 9.5
States and Political Subdivisions    
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]    
Balance, Beginning of Year 0.3 0.5
Additions for Securities for which No Previous Expected Credit Losses were Recognized 0.0 0.0
Reductions Due to Sales 0.0 0.0
Net (Decrease) Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized (0.2) (0.3)
Write-offs Charged Against Allowance 0.0  
Balance, End of Period 0.1 0.2
Corporate Bonds and Notes    
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]    
Balance, Beginning of Year 10.4 7.7
Additions for Securities for which No Previous Expected Credit Losses were Recognized 2.1 2.0
Reductions Due to Sales (0.4) (0.8)
Net (Decrease) Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized 4.7 0.4
Write-offs Charged Against Allowance (1.2)  
Balance, End of Period $ 15.6 $ 9.3