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Investments - Fair Value and Gross Unrealized Losses (Details)
$ in Millions
Mar. 31, 2024
USD ($)
Mar. 31, 2024
security
Mar. 31, 2024
Mar. 31, 2024
position
Dec. 31, 2023
USD ($)
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   298       195    
Number of positions with a gross unrealized loss, more than 12 months (security)   2,213   2,213   2,305   2,305
Number of positions with a gross unrealized loss, Total (security) | security   2,511       2,500    
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less (percent)     10.90%       9.20%  
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months (percent)     59.10%       65.20%  
Fair value as a percentage of total fixed maturities and equity securities fair value, Total (percent)     70.00%       74.40%  
Fixed maturity securities                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less $ 581.8       $ 479.2      
Fixed maturity securities, Fair Value, More than 12 Months 3,155.2       3,415.5      
Fixed maturity securities, Fair Value, Total 3,737.0       3,894.7      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 28.4       28.3      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 471.0       452.2      
Fixed maturity securities, Gross Unrealized Losses, Total 499.4       480.5      
Mortgage-backed securities                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 63.1       45.3      
Fixed maturity securities, Fair Value, More than 12 Months 470.4       458.5      
Fixed maturity securities, Fair Value, Total 533.5       503.8      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 1.0       0.8      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 70.7       63.8      
Fixed maturity securities, Gross Unrealized Losses, Total 71.7       64.6      
Other                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 100.8       39.5      
Fixed maturity securities, Fair Value, More than 12 Months 276.6       288.0      
Fixed maturity securities, Fair Value, Total 377.4       327.5      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 1.6       0.4      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 65.2       62.4      
Fixed maturity securities, Gross Unrealized Losses, Total 66.8       62.8      
Municipal bonds                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 111.7       64.5      
Fixed maturity securities, Fair Value, More than 12 Months 704.9       724.6      
Fixed maturity securities, Fair Value, Total 816.6       789.1      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 1.6       0.9      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 95.7       91.0      
Fixed maturity securities, Gross Unrealized Losses, Total 97.3       91.9      
Foreign government bonds                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 1.4       1.5      
Fixed maturity securities, Fair Value, More than 12 Months 20.5       20.6      
Fixed maturity securities, Fair Value, Total 21.9       22.1      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 0.1       0.0      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 1.1       1.0      
Fixed maturity securities, Gross Unrealized Losses, Total 1.2       1.0      
Corporate bonds                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 252.8       195.0      
Fixed maturity securities, Fair Value, More than 12 Months 1,147.4       1,171.3      
Fixed maturity securities, Fair Value, Total 1,400.2       1,366.3      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 23.5       25.4      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 206.2       194.9      
Fixed maturity securities, Gross Unrealized Losses, Total 229.7       220.3      
Other asset-backed securities                
Fixed maturity securities, Fair Value                
Fixed maturity securities, Fair Value, 12 Months or Less 52.0       133.4      
Fixed maturity securities, Fair Value, More than 12 Months 535.4       752.5      
Fixed maturity securities, Fair Value, Total 587.4       885.9      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less 0.6       0.8      
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months 32.1       39.1      
Fixed maturity securities, Gross Unrealized Losses, Total $ 32.7       $ 39.9