v3.26.1
Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and incurred losses and loss adjustment expenses (Detail) - Retroactive Reinsurance [Member] - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Liability for Claims and Claims Adjustment Expense [Line Items]    
Gross liabilities at the beginning of the period $ 31,048 $ 32,443
Losses and LAE incurred 428 352
Losses and LAE paid (1,068) (867)
Foreign currency effect (1) 128
Gross liabilities at end of period 29,978 31,733
Deferred Charges Reinsurance Assumed [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Losses and LAE incurred 429 323
Unpaid Losses and Loss Adjustment Expenses [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Losses and LAE incurred $ (1) $ 29