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Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($)
$ in Millions
Sep. 30, 2025
Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost $ 8,469.8 $ 7,730.9
Debt securities, Estimated fair value 8,248.2 7,265.9
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 48.8  
Amortized cost, Due after one through five years 159.4  
Amortized cost, Due after five through ten years 17.0  
Amortized cost, Due after ten years 24.2  
Debt Securities, Amortized cost 249.4 180.7
Estimated fair value, Due in one year or less 48.4  
Estimated fair value, Due after one through five years 159.3  
Estimated fair value, Due after five through ten years 17.1  
Estimated fair value, Due after ten years 24.1  
Debt securities, Estimated fair value 248.9 175.6
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 7.9  
Amortized cost, Due after one through five years 105.3  
Amortized cost, Due after five through ten years 375.9  
Amortized cost, Due after ten years 472.1  
Debt Securities, Amortized cost 961.2 844.9
Estimated fair value, Due in one year or less 7.9  
Estimated fair value, Due after one through five years 103.9  
Estimated fair value, Due after five through ten years 349.3  
Estimated fair value, Due after ten years 449.2  
Debt securities, Estimated fair value 910.3 776.5
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 24.8  
Amortized cost, Due after one through five years 148.7  
Amortized cost, Due after five through ten years 49.4  
Amortized cost, Due after ten years 3.6  
Debt Securities, Amortized cost 226.5 217.1
Estimated fair value, Due in one year or less 24.8  
Estimated fair value, Due after one through five years 147.9  
Estimated fair value, Due after five through ten years 48.0  
Estimated fair value, Due after ten years 2.9  
Debt securities, Estimated fair value 223.6 211.6
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 0.6  
Amortized cost, Due after one through five years 0.8  
Amortized cost, Due after five through ten years 207.6  
Amortized cost, Due after ten years 34.0  
Debt Securities, Amortized cost 243.0 203.8
Estimated fair value, Due in one year or less 0.6  
Estimated fair value, Due after one through five years 0.8  
Estimated fair value, Due after five through ten years 206.5  
Estimated fair value, Due after ten years 27.8  
Debt securities, Estimated fair value 235.7 189.8
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 11.6  
Amortized cost, Due after one through five years 544.1  
Amortized cost, Due after five through ten years 393.6  
Amortized cost, Due after ten years 99.5  
Debt Securities, Amortized cost 1,048.8 948.4
Estimated fair value, Due in one year or less 11.5  
Estimated fair value, Due after one through five years 548.7  
Estimated fair value, Due after five through ten years 398.7  
Estimated fair value, Due after ten years 91.3  
Debt securities, Estimated fair value 1,050.2 925.6
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 21.0  
Amortized cost, Due after one through five years 326.9  
Amortized cost, Due after five through ten years 103.5  
Amortized cost, Due after ten years 32.3  
Debt Securities, Amortized cost 483.7 491.6
Estimated fair value, Due in one year or less 21.0  
Estimated fair value, Due after one through five years 331.4  
Estimated fair value, Due after five through ten years 105.9  
Estimated fair value, Due after ten years 30.6  
Debt securities, Estimated fair value 488.9 $ 484.5
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 114.7  
Amortized cost, Due after one through five years 1,285.2  
Amortized cost, Due after five through ten years 1,147.0  
Amortized cost, Due after ten years 665.7  
Debt Securities, Amortized cost 3,212.6  
Estimated fair value, Due in one year or less 114.2  
Estimated fair value, Due after one through five years 1,292.0  
Estimated fair value, Due after five through ten years 1,125.5  
Estimated fair value, Due after ten years 625.9  
Debt securities, Estimated fair value 3,157.6  
Total Mortgage-Backed Securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 5,257.2  
Estimated fair value $ 5,090.6