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Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and incurred losses and loss adjustment expenses (Detail) - Retroactive Reinsurance [Member] - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]    
Gross liabilities at the beginning of the year $ 32,443 $ 34,647
Losses and loss adjustment expenses incurred 512 544
Losses and loss adjustment expenses paid (1,259) (1,596)
Foreign currency effect 110 75
Gross liabilities at end of period 31,307 33,060
Deferred Charges Reinsurance Assumed [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Losses and loss adjustment expenses incurred 499 610
Unpaid Losses and Loss Adjustment Expenses [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Losses and loss adjustment expenses incurred $ 13 $ (66)