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Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Unpaid Losses and Loss Expenses [Roll Forward] |  |  |  |  |  |
Gross unpaid losses and loss expenses, beginning of year |  | $ 75,747 | $ 72,330 |  | $ 67,192 |
Reinsurance recoverable on unpaid losses, beginning of year | [1] | (17,086) | (16,132) |  | (14,576) |
Net unpaid losses and loss expenses, beginning of year |  | 58,661 | 56,198 |  | 52,616 |
Net losses and loss expenses incurred in respect of losses occurring in Current Year |  | 24,956 | 23,680 |  | 21,986 |
Prior Year Claims and Claims Adjustment Expense, net of PPD related to A&H |  | (856) | (1,108) | [2] | (956) | [2]
Prior Year Claims and Claims Adjustment Expense |  | (882) |  |  |  |
Total |  | 24,100 | 22,572 |  | 21,030 |
Net losses and loss expenses paid in Current Year |  | 8,248 | 7,331 |  | 6,900 |
Net losses and loss expenses paid in Prior Year |  | 12,763 | 12,206 |  | 10,048 |
Total |  | 21,011 | 19,537 |  | 16,948 |
Foreign currency revaluation and other |  | 83 | (572) |  | (500) |
Net unpaid losses and loss expenses, end of year |  | 62,238 | 58,661 |  | 56,198 |
Reinsurance recoverable on unpaid losses, end of year | [1] | 17,884 | 17,086 |  | 16,132 |
Gross unpaid losses and loss expenses, end of year |  | 80,122 | 75,747 |  | 72,330 |
Prior Period Development, net adjustments |  | 83 | 232 |  | 30 |
Huatai Group [Member] |  |  |  |  |  |
Unpaid Losses and Loss Expenses [Roll Forward] |  |  |  |  |  |
Consolidation of Huatai Group |  | 405 | 0 |  | 0 |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Consolidation of Huatai Group |  | $ 405 | $ 0 |  | $ 0 |
[1] | Net of valuation allowance for uncollectible reinsurance.
[2] | Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments, earned premiums, and A&H long-duration lines totaling $83 million, $232 million, and $30 million for 2023, 2022, and 2021, respectively.

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