v3.26.1
Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost $ 10,062.0 $ 8,656.7
Debt securities, Estimated fair value 9,774.7 8,466.7
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 26.9  
Amortized cost, Due after one through five years 145.8  
Amortized cost, Due after five through ten years 45.1  
Amortized cost, Due after ten years 4.1  
Debt Securities, Amortized cost 221.9 244.2
Estimated fair value, Due in one year or less 26.7  
Estimated fair value, Due after one through five years 144.1  
Estimated fair value, Due after five through ten years 44.4  
Estimated fair value, Due after ten years 3.5  
Debt securities, Estimated fair value 218.7 243.6
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 1.2  
Amortized cost, Due after one through five years 143.8  
Amortized cost, Due after five through ten years 530.1  
Amortized cost, Due after ten years 424.8  
Debt Securities, Amortized cost 1,099.9 1,003.5
Estimated fair value, Due in one year or less 1.2  
Estimated fair value, Due after one through five years 141.3  
Estimated fair value, Due after five through ten years 495.2  
Estimated fair value, Due after ten years 415.6  
Debt securities, Estimated fair value 1,053.3 960.1
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 101.2  
Amortized cost, Due after one through five years 100.4  
Amortized cost, Due after five through ten years 20.9  
Amortized cost, Due after ten years 13.1  
Debt Securities, Amortized cost 235.6 239.4
Estimated fair value, Due in one year or less 101.7  
Estimated fair value, Due after one through five years 97.8  
Estimated fair value, Due after five through ten years 21.1  
Estimated fair value, Due after ten years 12.7  
Debt securities, Estimated fair value 233.3 235.2
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 226.4  
Amortized cost, Due after ten years 156.1  
Debt Securities, Amortized cost 383.9 268.3
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 224.0  
Estimated fair value, Due after ten years 149.2  
Debt securities, Estimated fair value 374.6 261.0
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 4.8  
Amortized cost, Due after one through five years 564.6  
Amortized cost, Due after five through ten years 527.9  
Amortized cost, Due after ten years 187.6  
Debt Securities, Amortized cost 1,284.9 1,032.8
Estimated fair value, Due in one year or less 4.7  
Estimated fair value, Due after one through five years 562.2  
Estimated fair value, Due after five through ten years 524.7  
Estimated fair value, Due after ten years 178.3  
Debt securities, Estimated fair value 1,269.9 1,034.8
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 18.1  
Amortized cost, Due after one through five years 272.4  
Amortized cost, Due after five through ten years 151.7  
Amortized cost, Due after ten years 53.7  
Debt Securities, Amortized cost 495.9 466.6
Estimated fair value, Due in one year or less 18.1  
Estimated fair value, Due after one through five years 272.8  
Estimated fair value, Due after five through ten years 152.0  
Estimated fair value, Due after ten years 51.4  
Debt securities, Estimated fair value 494.3 $ 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 152.8  
Amortized cost, Due after one through five years 1,227.8  
Amortized cost, Due after five through ten years 1,502.1  
Amortized cost, Due after ten years 839.4  
Debt Securities, Amortized cost 3,722.1  
Estimated fair value, Due in one year or less 153.0  
Estimated fair value, Due after one through five years 1,219.0  
Estimated fair value, Due after five through ten years 1,461.4  
Estimated fair value, Due after ten years 810.7  
Debt securities, Estimated fair value 3,644.1  
Total Mortgage-Backed Securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 6,339.9  
Estimated fair value $ 6,130.6