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Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2024
Dec. 31, 2023
Amortized Cost, net    
Due in 1 year or less $ 287.5  
Due after 1 year through 5 years 1,480.5  
Due after 5 years through 10 years 1,496.8  
Due after 10 years through 20 years 1,475.8  
Due after 20 years 1,044.8  
Amortized Cost, net 5,785.4 $ 5,652.9
Fair Value    
Due in 1 year or less 283.8  
Due after 1 year through 5 years 1,438.2  
Due after 5 years through 10 years 1,425.0  
Due after 10 years through 20 years 1,321.7  
Due after 20 years 874.0  
Total $ 5,342.7 $ 5,235.3
Percent of Total Fair Value    
Due in 1 year Or less (percent) 5.30%  
Due after 1 year through 5 years (percent) 26.90%  
Due after 5 years through 10 years (percent) 26.70%  
Due after 10 years through 20 years (percent) 24.70%  
Due after 20 years (percent) 16.40%  
Total (percent) 100.00%  
Average option-adjusted duration, in years 5 years 10 months 24 days