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Unpaid losses and loss expenses Unpaid losses and loss expenses (Reconciliation of reserve Balances to Liability for Unpaid Loss)(Details) (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | --- | ---
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | $ 50,492 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 14,687 |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Aggregate Reconciling Items | [1] | 1,109 |  |  |
Unpaid unallocated loss adjustment expenses |  | 1,523 |  |  |
Unpaid losses and loss expenses |  | 67,811 | $ 62,690 | $ 62,960 | $ 63,179
North America Commercial P&C Insurance - Workers' Compensation [Member] |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | 9,600 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 1,461 |  |  |
North America Commercial P&C Insurance - Liability [Member] |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | 17,441 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 5,743 |  |  |
North America Commercial P&C Insurance - Other Casualty [Member] |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | 2,124 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 564 |  |  |
North America Commercial P&C Insurance - Non-Casualty [Member] |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | 2,491 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 1,380 |  |  |
North America Personal P&C Insurance [Member] |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | 3,025 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 141 |  |  |
Overseas General Insurance - Casualty [Member] |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | 6,832 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 2,426 |  |  |
Overseas General Insurance - Non-Casualty [Member] |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | 2,725 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 1,686 |  |  |
Global Reinsurance - Casualty [Member] |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | 1,200 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 29 |  |  |
Global Reinsurance - Non-Casualty [Member] |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | 312 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 104 |  |  |
Other Segments [Member] |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | 4,742 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | $ 1,153 |  |  |
[1] | Primarily includes the claims reserve of our International A&H business and Life Insurance segment reserves.

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