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Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($)
$ in Millions
Sep. 30, 2024
Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost $ 7,256.7 $ 7,895.2
Debt securities, Estimated fair value 7,015.3 7,157.5
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 40.6  
Amortized cost, Due after one through five years 133.8  
Amortized cost, Due after five through ten years 4.7  
Amortized cost, Due after ten years 12.7  
Debt Securities, Amortized cost 191.8 203.3
Estimated fair value, Due in one year or less 40.4  
Estimated fair value, Due after one through five years 132.7  
Estimated fair value, Due after five through ten years 4.9  
Estimated fair value, Due after ten years 12.4  
Debt securities, Estimated fair value 190.4 199.3
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 8.6  
Amortized cost, Due after one through five years 52.2  
Amortized cost, Due after five through ten years 231.6  
Amortized cost, Due after ten years 484.8  
Debt Securities, Amortized cost 777.2 1,373.7
Estimated fair value, Due in one year or less 8.6  
Estimated fair value, Due after one through five years 50.9  
Estimated fair value, Due after five through ten years 207.7  
Estimated fair value, Due after ten years 464.2  
Debt securities, Estimated fair value 731.4 1,245.8
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 17.0  
Amortized cost, Due after one through five years 136.7  
Amortized cost, Due after five through ten years 70.3  
Amortized cost, Due after ten years 6.7  
Debt Securities, Amortized cost 230.7 228.4
Estimated fair value, Due in one year or less 16.9  
Estimated fair value, Due after one through five years 138.8  
Estimated fair value, Due after five through ten years 64.8  
Estimated fair value, Due after ten years 6.1  
Debt securities, Estimated fair value 226.6 219.3
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 8.5  
Amortized cost, Due after one through five years 76.4  
Amortized cost, Due after five through ten years 99.8  
Amortized cost, Due after ten years 44.9  
Debt Securities, Amortized cost 229.6 207.7
Estimated fair value, Due in one year or less 8.5  
Estimated fair value, Due after one through five years 76.4  
Estimated fair value, Due after five through ten years 99.3  
Estimated fair value, Due after ten years 37.8  
Debt securities, Estimated fair value 222.0 195.4
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 13.6  
Amortized cost, Due after one through five years 495.3  
Amortized cost, Due after five through ten years 272.0  
Amortized cost, Due after ten years 84.3  
Debt Securities, Amortized cost 865.2 1,007.0
Estimated fair value, Due in one year or less 13.6  
Estimated fair value, Due after one through five years 497.7  
Estimated fair value, Due after five through ten years 275.0  
Estimated fair value, Due after ten years 78.2  
Debt securities, Estimated fair value 864.5 958.4
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 34.2  
Amortized cost, Due after one through five years 299.7  
Amortized cost, Due after five through ten years 129.7  
Amortized cost, Due after ten years 29.8  
Debt Securities, Amortized cost 493.4 478.9
Estimated fair value, Due in one year or less 34.0  
Estimated fair value, Due after one through five years 301.9  
Estimated fair value, Due after five through ten years 130.8  
Estimated fair value, Due after ten years 27.8  
Debt securities, Estimated fair value 494.5 $ 463.6
Aggregate Debt Securities Excluding Mortgage Backed And Asset Backed Securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 122.5  
Amortized cost, Due after one through five years 1,194.1  
Amortized cost, Due after five through ten years 808.1  
Amortized cost, Due after ten years 663.2  
Debt Securities, Amortized cost 2,787.9  
Estimated fair value, Due in one year or less 122.0  
Estimated fair value, Due after one through five years 1,198.4  
Estimated fair value, Due after five through ten years 782.5  
Estimated fair value, Due after ten years 626.5  
Debt securities, Estimated fair value 2,729.4  
Total Mortgage-Backed Securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 4,468.8  
Estimated fair value $ 4,285.9