v3.26.3
Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost $ 10,535.2 $ 8,656.7
Debt securities, Estimated fair value 9,762.6 8,466.7
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 39.8  
Amortized cost, Due after one through five years 162.2  
Amortized cost, Due after five through ten years 50.9  
Amortized cost, Due after ten years 4.1  
Debt Securities, Amortized cost 257.0 244.2
Estimated fair value, Due in one year or less 39.7  
Estimated fair value, Due after one through five years 157.1  
Estimated fair value, Due after five through ten years 47.6  
Estimated fair value, Due after ten years 3.1  
Debt securities, Estimated fair value 247.5 243.6
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 7.2  
Amortized cost, Due after one through five years 149.4  
Amortized cost, Due after five through ten years 574.0  
Amortized cost, Due after ten years 415.7  
Debt Securities, Amortized cost 1,146.3 1,003.5
Estimated fair value, Due in one year or less 7.2  
Estimated fair value, Due after one through five years 141.3  
Estimated fair value, Due after five through ten years 509.4  
Estimated fair value, Due after ten years 371.3  
Debt securities, Estimated fair value 1,029.2 960.1
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 93.2  
Amortized cost, Due after one through five years 101.8  
Amortized cost, Due after five through ten years 33.6  
Amortized cost, Due after ten years 13.2  
Debt Securities, Amortized cost 241.8 239.4
Estimated fair value, Due in one year or less 93.4  
Estimated fair value, Due after one through five years 98.0  
Estimated fair value, Due after five through ten years 33.1  
Estimated fair value, Due after ten years 11.8  
Debt securities, Estimated fair value 236.3 235.2
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 0.0  
Amortized cost, Due after one through five years 0.8  
Amortized cost, Due after five through ten years 272.9  
Amortized cost, Due after ten years 200.6  
Debt Securities, Amortized cost 474.3 268.3
Estimated fair value, Due in one year or less 0.0  
Estimated fair value, Due after one through five years 0.8  
Estimated fair value, Due after five through ten years 259.8  
Estimated fair value, Due after ten years 183.4  
Debt securities, Estimated fair value 444.0 261.0
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 3.1  
Amortized cost, Due after one through five years 590.7  
Amortized cost, Due after five through ten years 533.2  
Amortized cost, Due after ten years 172.1  
Debt Securities, Amortized cost 1,299.1 1,032.8
Estimated fair value, Due in one year or less 3.1  
Estimated fair value, Due after one through five years 576.8  
Estimated fair value, Due after five through ten years 505.3  
Estimated fair value, Due after ten years 149.1  
Debt securities, Estimated fair value 1,234.3 1,034.8
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 17.3  
Amortized cost, Due after one through five years 274.6  
Amortized cost, Due after five through ten years 152.6  
Amortized cost, Due after ten years 49.0  
Debt Securities, Amortized cost 493.5 466.6
Estimated fair value, Due in one year or less 17.2  
Estimated fair value, Due after one through five years 269.8  
Estimated fair value, Due after five through ten years 146.5  
Estimated fair value, Due after ten years 43.6  
Debt securities, Estimated fair value 477.1 $ 471.2
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 160.6  
Amortized cost, Due after one through five years 1,279.5  
Amortized cost, Due after five through ten years 1,617.2  
Amortized cost, Due after ten years 854.7  
Debt Securities, Amortized cost 3,912.0  
Estimated fair value, Due in one year or less 160.6  
Estimated fair value, Due after one through five years 1,243.8  
Estimated fair value, Due after five through ten years 1,501.7  
Estimated fair value, Due after ten years 762.3  
Debt securities, Estimated fair value 3,668.4  
Total Mortgage-Backed Securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 6,623.2  
Estimated fair value $ 6,094.2