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Claim and Claim Adjustment Expense Reserves (Specialty - Line of Business Composition) (Details) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
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Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total net liability for unpaid claim and claim adjustment expenses | $ 19,263 | $ 18,163 | $ 16,929 | $ 16,300
Specialty |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total net liability for unpaid claim and claim adjustment expenses | 5,979 | $ 5,916 |  |
Medical Professional Liability | Specialty |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total net liability for unpaid claim and claim adjustment expenses | 1,425 |  |  |
Other Professional Liability and Management Liability | Specialty |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total net liability for unpaid claim and claim adjustment expenses | 3,967 |  |  |
Surety | Specialty |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total net liability for unpaid claim and claim adjustment expenses | 493 |  |  |
Warranty | Specialty |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total net liability for unpaid claim and claim adjustment expenses | 46 |  |  |
Other | Specialty |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total net liability for unpaid claim and claim adjustment expenses | $ 48 |  |  |

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