| **Claim and Claim Adjustment Expense Reserves (International - Detail of Development Recorded) (Details) - International - USD ($) $ in Millions** | 3 Months Ended | 6 Months Ended |
|----------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                                   | $ (2)          | $ 2            | $ (2) | $ 0 |
| Casualty                                                                                                                                           |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                                   | (6)            | 1              | (6) | 1 |
| Property                                                                                                                                           |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                                   | 12             | (10)           | 12 | (11) |
| Energy and Marine                                                                                                                                  |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                                   | (5)            | (3)            | (5) | (3) |
| Specialty                                                                                                                                          |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                                   | 9              | 16             | 9 | 15 |
| Healthcare and Technology                                                                                                                          |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                                   | $ (12)         | $ (2)          | $ (12) | $ (2) |

| 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_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after effects of reinsurance, of increase (decrease) in the liability for unpaid claims and claims adjustment expense.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437
+ Details**Name:**us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**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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| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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