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Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2024
Dec. 31, 2023
Amortized Cost, net    
Due in 1 year or less $ 267.8  
Due after 1 year through 5 years 1,531.5  
Due after 5 years through 10 years 1,597.0  
Due after 10 years through 20 years 1,452.2  
Due after 20 years 1,026.5  
Amortized Cost, net 5,875.0 $ 5,652.9
Fair Value    
Due in 1 year or less 268.4  
Due after 1 year through 5 years 1,518.5  
Due after 5 years through 10 years 1,548.1  
Due after 10 years through 20 years 1,345.4  
Due after 20 years 892.8  
Total $ 5,573.2 $ 5,235.3
Percent of Total Fair Value    
Due in 1 year Or less (percent) 4.80%  
Due after 1 year through 5 years (percent) 27.20%  
Due after 5 years through 10 years (percent) 27.80%  
Due after 10 years through 20 years (percent) 24.20%  
Due after 20 years (percent) 16.00%  
Total (percent) 100.00%  
Average option-adjusted duration, in years 5 years 8 months 12 days