| **Claim and Claim Adjustment Expense Reserves (Commercial - Detail of Develpment Recorded) (Details) - Commercial - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                         |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                            | $ (5)          | $ (17)         | $ (27) | $ (94) |
| Commercial Auto                                                                                                                             |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                         |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                            | 1              | (12)           | 0 | (37) |
| General Liability                                                                                                                           |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                         |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                            | (5)            | (2)            | 13 | (19) |
| Workers' Compensation                                                                                                                       |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                         |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                            | (2)            | 9              | (14) | (38) |
| Property and Other                                                                                                                          |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                         |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                            | $ 1            | $ (12)         | $ (26) | $ 0 |

| 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.
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| 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
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| X                                                                                                                                                  |
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