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Unpaid losses and loss expenses Unpaid losses and loss expenses (Reconciliation of reserve Balances to Liability for Unpaid Loss) (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | --- | --- | ---
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Net Unpaid Loss and Allocated Loss Adjustment Expense | $ 60,209 |  |  |
Ceded unpaid loss and allocated loss adjustment expense | 18,156 |  |  |
Unpaid unallocated loss adjustment expenses | 1,757 |  |  |
Unpaid losses and loss expenses | 80,122 | $ 75,747 | $ 72,330 | $ 67,192
North America Commercial P&C Insurance - Workers' Compensation [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Net Unpaid Loss and Allocated Loss Adjustment Expense | 10,159 |  |  |
Ceded unpaid loss and allocated loss adjustment expense | 1,190 |  |  |
North America Commercial P&C Insurance - Liability [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Net Unpaid Loss and Allocated Loss Adjustment Expense | 21,092 |  |  |
Ceded unpaid loss and allocated loss adjustment expense | 7,427 |  |  |
North America Commercial P&C Insurance - Other Casualty [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Net Unpaid Loss and Allocated Loss Adjustment Expense | 2,596 |  |  |
Ceded unpaid loss and allocated loss adjustment expense | 1,064 |  |  |
North America Commercial P&C Insurance - Non-Casualty [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Net Unpaid Loss and Allocated Loss Adjustment Expense | 3,203 |  |  |
Ceded unpaid loss and allocated loss adjustment expense | 1,673 |  |  |
North America Personal P&C Insurance [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Net Unpaid Loss and Allocated Loss Adjustment Expense | 3,550 |  |  |
Ceded unpaid loss and allocated loss adjustment expense | 577 |  |  |
Overseas General Insurance - Casualty [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Net Unpaid Loss and Allocated Loss Adjustment Expense | 8,157 |  |  |
Ceded unpaid loss and allocated loss adjustment expense | 2,527 |  |  |
Overseas General Insurance - Non-Casualty [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Net Unpaid Loss and Allocated Loss Adjustment Expense | 3,515 |  |  |
Ceded unpaid loss and allocated loss adjustment expense | 1,736 |  |  |
Global Reinsurance - Casualty [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Net Unpaid Loss and Allocated Loss Adjustment Expense | 1,240 |  |  |
Ceded unpaid loss and allocated loss adjustment expense | 80 |  |  |
Global Reinsurance - Non-Casualty [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Net Unpaid Loss and Allocated Loss Adjustment Expense | 411 |  |  |
Ceded unpaid loss and allocated loss adjustment expense | 101 |  |  |
Other Segments [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Net Unpaid Loss and Allocated Loss Adjustment Expense | 6,286 |  |  |
Ceded unpaid loss and allocated loss adjustment expense | $ 1,781 |  |  |

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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (cc) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 + Details
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