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, 2025 |
Jun. 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 | 352 | 363 |
| Losses and loss adjustment expenses paid | (867) | (1,066) |
| Foreign currency effect | 128 | (19) |
| Gross liabilities at end of period | 31,733 | 33,494 |
| Deferred Charges Reinsurance Assumed [Member] | ||
| Liability for Claims and Claims Adjustment Expense [Line Items] | ||
| Losses and loss adjustment expenses incurred | 323 | 431 |
| Unpaid Losses and Loss Adjustment Expenses [Member] | ||
| Liability for Claims and Claims Adjustment Expense [Line Items] | ||
| Losses and loss adjustment expenses incurred | $ 29 | $ (68) |