| **Legal Proceedings, Contingencies and Guarantees (Narrative) (Details) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |         |
|---------------------------------------------------------------------------------------------------|----------------|----------------|---------|
||
| Guarantee Obligations                                                                             |           |           |    |   |   |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**                 |           |           |    |   |   |
| Guarantor obligations, maximum exposure, undiscounted                                             | $ 1,800        |           | $ 1,800 |   |   |
| Guarantee and Indemnification Agreements                                                          |           |           |    |   |   |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**                 |           |           |    |   |   |
| Guarantor obligations, current carrying calue                                                     | 5              |           | 5       |   | $ 5 |
| Guarantee Agreement                                                                               |           |           |    |   |   |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**                 |           |           |    |   |   |
| Guarantor obligations, maximum exposure, undiscounted                                             | 375            |           | 375     |   |   |
| Indemnification Agreement                                                                         |           |           |    |   |   |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**                 |           |           |    |   |   |
| Guarantor obligations, maximum exposure, undiscounted                                             | 252            |           | 252     |   |   |
| Earned Premiums | Insurance Premium Rate Adjustment | Multi-Peril Package Product                 |           |           |    |   |   |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**                 |           |           |    |   |   |
| Earned premium adjustment                                                                         | $ 6            |           | 6       | $ (37) |   |
| Interest Expense | Insurance Premium Rate Adjustment | Multi-Peril Package Product                |           |           |    |   |   |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**                 |           |           |    |   |   |
| Premium rate adjustment                                                                           |           | $ 1            | $ 1     | $ 7 |   |

| X                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
[Line Items] for Legal Proceedings, Commitments and Contingencies, and Guarantees [Table]
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of increase (decrease) in loss contingency liability.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount charged against operating income increasing loss contingency liability, after adjustments to reduce previously estimated charges.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349
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