| **Claim and Claim Adjustment Expense Reserves (Specialty - Line of Business Composition) (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]**                       |          |          |          |          |
| Total net liability for unpaid claim and claim adjustment expenses                                                           | $ 17,965      | $ 18,070      | $ 18,249      | $ 18,576      |
| Specialty                                                                                                                    |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                       |          |          |          |          |
| Total net liability for unpaid claim and claim adjustment expenses                                                           | 4,831         | $ 5,007       |          |          |
| 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                                                           | 1,457         |          |          |          |
| 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                                                           | 2,849         |          |          |          |
| Surety | Specialty                                                                                                           |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                       |          |          |          |          |
| Total net liability for unpaid claim and claim adjustment expenses                                                           | 379           |          |          |          |
| Warranty | Specialty                                                                                                         |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                       |          |          |          |          |
| Total net liability for unpaid claim and claim adjustment expenses                                                           | 40            |          |          |          |
| Other | Specialty                                                                                                            |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                       |          |          |          |          |
| Total net liability for unpaid claim and claim adjustment expenses                                                           | $ 106         |          |          |          |

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| - Definition
Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims.
+ References
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| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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