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Short Duration Contracts - Reconciliation of claims development to liability (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
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Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Liabilities for losses and loss adjustment expenses, net of reinsurance |  | $ 15,700 |  |  |
Unpaid losses and loss adjustment expenses recoverable |  | 6,690 | $ 6,280 | $ 5,599 | $ 4,315
Lines other than short duration |  | 132 |  |  |
Discounting |  | (66) |  |  |
Unallocated claims adjustment expenses |  | 296 |  |  |
Total reconciling items |  | 362 |  |  |
Reserve for losses and loss adjustment expenses |  | 22,752 | $ 20,032 | $ 17,757 | $ 16,514
Other short duration lines not included |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Liabilities for losses and loss adjustment expenses, net of reinsurance |  | 284 |  |  |
Unpaid losses and loss adjustment expenses recoverable | [1] | 468 |  |  |
Other short duration lines not included | Loss portfolio transfer |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid losses and loss adjustment expenses recoverable | [1] | 225 |  |  |
Total for short duration lines |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid losses and loss adjustment expenses recoverable |  | 6,690 |  |  |
Insurance | Property, energy, marine and aviation |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Liabilities for losses and loss adjustment expenses, net of reinsurance |  | 824 |  |  |
Unpaid losses and loss adjustment expenses recoverable |  | 359 |  |  |
Insurance | Third party occurrence business |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Liabilities for losses and loss adjustment expenses, net of reinsurance |  | 3,601 |  |  |
Unpaid losses and loss adjustment expenses recoverable |  | 2,025 |  |  |
Insurance | Third party claims-made business |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Liabilities for losses and loss adjustment expenses, net of reinsurance |  | 2,352 |  |  |
Unpaid losses and loss adjustment expenses recoverable |  | 1,001 |  |  |
Insurance | Multi-line and other specialty |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Liabilities for losses and loss adjustment expenses, net of reinsurance |  | 1,348 |  |  |
Unpaid losses and loss adjustment expenses recoverable |  | 246 |  |  |
Reinsurance | Casualty |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Liabilities for losses and loss adjustment expenses, net of reinsurance |  | 2,706 |  |  |
Unpaid losses and loss adjustment expenses recoverable |  | 738 |  |  |
Reinsurance | Property catastrophe |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Liabilities for losses and loss adjustment expenses, net of reinsurance |  | 563 |  |  |
Unpaid losses and loss adjustment expenses recoverable |  | 519 |  |  |
Reinsurance | Property excluding property catastrophe |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Liabilities for losses and loss adjustment expenses, net of reinsurance |  | 1,261 |  |  |
Unpaid losses and loss adjustment expenses recoverable |  | 239 |  |  |
Reinsurance | Marine and aviation |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Liabilities for losses and loss adjustment expenses, net of reinsurance |  | 357 |  |  |
Unpaid losses and loss adjustment expenses recoverable |  | 240 |  |  |
Reinsurance | Other specialty |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Liabilities for losses and loss adjustment expenses, net of reinsurance |  | 2,094 |  |  |
Unpaid losses and loss adjustment expenses recoverable |  | 828 |  |  |
Mortgage |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Liabilities for losses and loss adjustment expenses, net of reinsurance |  | 310 |  |  |
Unpaid losses and loss adjustment expenses recoverable |  | 33 |  |  |
Consolidating adjustments and eliminations |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |
Unpaid losses and loss adjustment expenses recoverable |  | $ (6) |  |  |
[1] | Includes unpaid loss and loss adjustment expenses recoverable of $225 million related to the loss portfolio transfer reinsurance agreements.

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