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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, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | --- | ---
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | $ 45,847 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 14,857 |  |  |
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Aggregate Reconciling Items | [1] | 831 |  |  |
Unpaid unallocated loss adjustment expenses |  | 1,425 |  |  |
Unpaid losses and loss expenses |  | 62,960 | $ 63,179 | $ 60,540 | $ 37,303
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 |  | 9,183 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 1,766 |  |  |
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 |  | 16,485 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 4,812 |  |  |
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 |  | 1,884 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 544 |  |  |
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 |  | 1,871 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 1,531 |  |  |
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 |  | 2,319 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 895 |  |  |
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 |  | 5,833 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 2,070 |  |  |
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 |  | 2,265 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 1,208 |  |  |
Global Reinsurance - Casualty [Member] |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | 1,218 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 58 |  |  |
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 |  | 390 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | 99 |  |  |
Other Short-duration Insurance Product Line [Member] |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid Loss and Allocated Loss Adjustment Expenses, Net |  | 4,399 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments |  | $ 1,874 |  |  |
[1] | Primarily includes the claims reserve of our International A&H business and Life Insurance segment reserves.

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