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Loss and Loss Adjustment Expense Reserves (Loss and LAE Activity) (Narrative) (Detail) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Dec. 31, 2024
Loss And Loss Adjustment Expense Reserves [Line Items]      
Weighted average risk-free rates used to discount claim liability 3.89%   4.50%
Lae [Member]      
Loss And Loss Adjustment Expense Reserves [Line Items]      
Losses and loss adjustment $ 3 $ 11  
Loss and loss adjustment expense reserves $ 13   $ 19
Changes in Loss and LAE Reserves one-year