v3.26.3
Investments - Fair Value and Gross Unrealized Losses (Details)
$ in Millions
Sep. 30, 2026
USD ($)
Sep. 30, 2026
security
Sep. 30, 2026
Sep. 30, 2026
position
Dec. 31, 2025
USD ($)
Dec. 31, 2025
security
Dec. 31, 2025
Dec. 31, 2025
position
Fixed maturity securities, Gross Unrealized Losses                
Number of positions with a gross unrealized loss, 12 months or less (security) | security   1,631       466    
Number of positions with a gross unrealized loss, more than 12 months (security)   1,540   1,540   1,649   1,649
Number of positions with a gross unrealized loss, total (security) | security   3,171       2,115    
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 months or less (percent)     43.30%       14.90%  
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months (percent)     37.60%       42.50%  
Fair value as a percentage of total fixed maturities and equity securities fair value, total (percent)     80.90%       57.40%  
Fixed maturity securities                
Fixed maturity securities, Fair Value                
Fixed maturity securities, fair value, 12 months or less $ 2,387.9       $ 849.6      
Fixed maturity securities, fair value, more than 12 months 2,075.1       2,429.8      
Fixed maturity securities, fair value, total 4,463.0       3,279.4      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, gross unrealized losses, 12 months or less 125.6       15.2      
Fixed maturity securities, gross unrealized losses, more than 12 months 521.2       371.0      
Fixed maturity securities, gross unrealized losses, total 646.8       386.2      
Mortgage-backed securities                
Fixed maturity securities, Fair Value                
Fixed maturity securities, fair value, 12 months or less 432.9       69.7      
Fixed maturity securities, fair value, more than 12 months 210.6       247.9      
Fixed maturity securities, fair value, total 643.5       317.6      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, gross unrealized losses, 12 months or less 22.3       0.3      
Fixed maturity securities, gross unrealized losses, more than 12 months 52.4       38.3      
Fixed maturity securities, gross unrealized losses, total 74.7       38.6      
Other                
Fixed maturity securities, Fair Value                
Fixed maturity securities, fair value, 12 months or less 47.7       27.1      
Fixed maturity securities, fair value, more than 12 months 249.9       276.1      
Fixed maturity securities, fair value, total 297.6       303.2      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, gross unrealized losses, 12 months or less 3.7       0.4      
Fixed maturity securities, gross unrealized losses, more than 12 months 75.3       55.7      
Fixed maturity securities, gross unrealized losses, total 79.0       56.1      
Municipal bonds                
Fixed maturity securities, Fair Value                
Fixed maturity securities, fair value, 12 months or less 418.3       126.0      
Fixed maturity securities, fair value, more than 12 months 529.6       596.4      
Fixed maturity securities, fair value, total 947.9       722.4      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, gross unrealized losses, 12 months or less 23.2       2.8      
Fixed maturity securities, gross unrealized losses, more than 12 months 122.8       79.9      
Fixed maturity securities, gross unrealized losses, total 146.0       82.7      
Foreign government bonds                
Fixed maturity securities, Fair Value                
Fixed maturity securities, fair value, 12 months or less 0.9       0.0      
Fixed maturity securities, fair value, more than 12 months 6.7       9.0      
Fixed maturity securities, fair value, total 7.6       9.0      
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 1.0       0.6      
Fixed maturity securities, gross unrealized losses, total 1.0       0.6      
Corporate bonds                
Fixed maturity securities, Fair Value                
Fixed maturity securities, fair value, 12 months or less 854.8       387.3      
Fixed maturity securities, fair value, more than 12 months 819.7       930.6      
Fixed maturity securities, fair value, total 1,674.5       1,317.9      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, gross unrealized losses, 12 months or less 59.8       10.5      
Fixed maturity securities, gross unrealized losses, more than 12 months 235.2       171.6      
Fixed maturity securities, gross unrealized losses, total 295.0       182.1      
Other asset-backed securities                
Fixed maturity securities, Fair Value                
Fixed maturity securities, fair value, 12 months or less 633.3       239.5      
Fixed maturity securities, fair value, more than 12 months 258.6       369.8      
Fixed maturity securities, fair value, total 891.9       609.3      
Fixed maturity securities, Gross Unrealized Losses                
Fixed maturity securities, gross unrealized losses, 12 months or less 16.6       1.2      
Fixed maturity securities, gross unrealized losses, more than 12 months 34.5       24.9      
Fixed maturity securities, gross unrealized losses, total $ 51.1       $ 26.1