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Investments - Fair Value and Gross Unrealized Losses (Details)
$ in Millions
Sep. 30, 2025
USD ($)
Sep. 30, 2025
security
Sep. 30, 2025
Sep. 30, 2025
position
Dec. 31, 2024
USD ($)
Dec. 31, 2024
security
Dec. 31, 2024
Dec. 31, 2024
position
Fixed maturity securities, Gross Unrealized Losses                
Number of positions with a gross unrealized loss, 12 Months or Less (security) | security   427       665    
Number of positions with a gross unrealized loss, more than 12 months (security)   1,708   1,708   1,862   1,862
Number of positions with a gross unrealized loss, Total (security) | security   2,135       2,527    
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less (percent)     10.30%       15.80%  
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months (percent)     44.30%       51.10%  
Fair value as a percentage of total fixed maturities and equity securities fair value, Total (percent)     54.60%       66.90%  
Fixed maturity securities                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less $ 585.5       $ 849.1      
Fixed maturity securities, Fair Value, More than 12 Months 2,509.5       2,751.7      
Fixed maturity securities, Fair Value, Total 3,095.0       3,600.8      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 12.5       19.2      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 382.7       483.8      
Fixed maturity securities, Gross Unrealized Losses, Total 395.2       503.0      
Mortgage-backed securities                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 43.8       208.3      
Fixed maturity securities, Fair Value, More than 12 Months 244.3       419.3      
Fixed maturity securities, Fair Value, Total 288.1       627.6      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 0.4       3.1      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 39.2       71.7      
Fixed maturity securities, Gross Unrealized Losses, Total 39.6       74.8      
Other                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 10.7       77.1      
Fixed maturity securities, Fair Value, More than 12 Months 285.6       274.6      
Fixed maturity securities, Fair Value, Total 296.3       351.7      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 0.2       1.5      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 56.4       67.6      
Fixed maturity securities, Gross Unrealized Losses, Total 56.6       69.1      
Municipal bonds                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 190.3       193.5      
Fixed maturity securities, Fair Value, More than 12 Months 616.2       632.0      
Fixed maturity securities, Fair Value, Total 806.5       825.5      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 4.9       3.4      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 83.1       102.4      
Fixed maturity securities, Gross Unrealized Losses, Total 88.0       105.8      
Foreign government bonds                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 0.0       1.5      
Fixed maturity securities, Fair Value, More than 12 Months 12.0       11.7      
Fixed maturity securities, Fair Value, Total 12.0       13.2      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 0.0       0.0      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 0.6       0.9      
Fixed maturity securities, Gross Unrealized Losses, Total 0.6       0.9      
Corporate bonds                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 162.9       235.4      
Fixed maturity securities, Fair Value, More than 12 Months 954.6       1,075.8      
Fixed maturity securities, Fair Value, Total 1,117.5       1,311.2      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 5.9       10.3      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 175.7       219.8      
Fixed maturity securities, Gross Unrealized Losses, Total 181.6       230.1      
Other asset-backed securities                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 177.8       133.3      
Fixed maturity securities, Fair Value, More than 12 Months 396.8       338.3      
Fixed maturity securities, Fair Value, Total 574.6       471.6      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 1.1       0.9      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 27.7       21.4      
Fixed maturity securities, Gross Unrealized Losses, Total $ 28.8       $ 22.3