| **Claim and Claim Adjustment Expense Reserves (Net Prior Year Development) (Details) - USD ($) $ in Millions** | 12 Months Ended |
|----------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Segment Reporting Information [Line Items]**                                                                 |            |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                 | $ 48            | $ (32) | $ 11 |
| Specialty                                                                                                      |            |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |            |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                 | (14)            | (40) | (45) |
| Commercial                                                                                                     |            |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |            |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                 | (22)            | (43) | (6) |
| International                                                                                                  |            |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |            |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                 | 13              | (13) | 2 |
| Corporate & Other                                                                                          |            |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |            |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                 | $ 71            | $ 64 | $ 60 |

| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax Favorable Unfavorable Premium Development, excluding Life & Group
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ Details**Name:**us-gaap_SegmentReportingInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                         |
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| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                             |
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