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Investments - Fair Value and Gross Unrealized Losses (Details)
$ in Millions
Sep. 30, 2024
USD ($)
Sep. 30, 2024
security
Sep. 30, 2024
Sep. 30, 2024
position
Dec. 31, 2023
USD ($)
Dec. 31, 2023
security
Dec. 31, 2023
Dec. 31, 2023
position
Fixed maturity securities, Gross Unrealized Losses                
Number of positions with a gross unrealized loss, 12 Months or Less (security) | security   163       195    
Number of positions with a gross unrealized loss, more than 12 months (security)   1,953   1,953   2,305   2,305
Number of positions with a gross unrealized loss, Total (security) | security   2,116       2,500    
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less (percent)     5.40%       9.20%  
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months (percent)     53.60%       65.20%  
Fair value as a percentage of total fixed maturities and equity securities fair value, Total (percent)     59.00%       74.40%  
Fixed maturity securities                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less $ 298.3       $ 479.2      
Fixed maturity securities, Fair Value, More than 12 Months 2,988.1       3,415.5      
Fixed maturity securities, Fair Value, Total 3,286.4       3,894.7      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 10.9       28.3      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 377.9       452.2      
Fixed maturity securities, Gross Unrealized Losses, Total 388.8       480.5      
Mortgage-backed securities                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 0.0       45.3      
Fixed maturity securities, Fair Value, More than 12 Months 471.4       458.5      
Fixed maturity securities, Fair Value, Total 471.4       503.8      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 0.0       0.8      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 51.3       63.8      
Fixed maturity securities, Gross Unrealized Losses, Total 51.3       64.6      
Other                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 1.3       39.5      
Fixed maturity securities, Fair Value, More than 12 Months 292.8       288.0      
Fixed maturity securities, Fair Value, Total 294.1       327.5      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 0.0       0.4      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 51.9       62.4      
Fixed maturity securities, Gross Unrealized Losses, Total 51.9       62.8      
Municipal bonds                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 19.3       64.5      
Fixed maturity securities, Fair Value, More than 12 Months 681.1       724.6      
Fixed maturity securities, Fair Value, Total 700.4       789.1      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 0.1       0.9      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 72.5       91.0      
Fixed maturity securities, Gross Unrealized Losses, Total 72.6       91.9      
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 14.9       20.6      
Fixed maturity securities, Fair Value, Total 14.9       22.1      
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 0.7       1.0      
Fixed maturity securities, Gross Unrealized Losses, Total 0.7       1.0      
Corporate bonds                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 177.7       195.0      
Fixed maturity securities, Fair Value, More than 12 Months 1,136.1       1,171.3      
Fixed maturity securities, Fair Value, Total 1,313.8       1,366.3      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 10.6       25.4      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 182.0       194.9      
Fixed maturity securities, Gross Unrealized Losses, Total 192.6       220.3      
Other asset-backed securities                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 100.0       133.4      
Fixed maturity securities, Fair Value, More than 12 Months 391.8       752.5      
Fixed maturity securities, Fair Value, Total 491.8       885.9      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 0.2       0.8      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 19.5       39.1      
Fixed maturity securities, Gross Unrealized Losses, Total $ 19.7       $ 39.9