Retroactive reinsurance contracts - Reconciliation of changes in claim liabilities and incurred losses and loss adjustment expenses (Detail) - Retroactive Reinsurance [Member] - USD ($) $ in Millions |
3 Months Ended | |
|---|---|---|
Mar. 31, 2026 |
Mar. 31, 2025 |
|
| Liability for Claims and Claims Adjustment Expense [Line Items] | ||
| Gross liabilities at the beginning of the year | $ 31,048 | $ 32,443 |
| Losses and LAE incurred | 251 | 171 |
| Losses and LAE paid | (674) | (511) |
| Foreign currency effect | (5) | 40 |
| Gross liabilities at end of year | 30,368 | 31,974 |
| Deferred Charges Reinsurance Assumed [Member] | ||
| Liability for Claims and Claims Adjustment Expense [Line Items] | ||
| Losses and LAE incurred | 252 | 169 |
| Unpaid Losses and Loss Adjustment Expenses [Member] | ||
| Liability for Claims and Claims Adjustment Expense [Line Items] | ||
| Losses and LAE incurred | $ (1) | $ 2 |