| **Property Casualty Loss And Loss Expenses (Claims Development - Commercial Auto) (Details) reported_claim in Thousands, $ in Millions** | Dec. 31, 2016 USD ($) reported_claim | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($) |
|------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------|-----------------------|-----------------------|-----------------------|-----------------------|
| **Claims Development [Line Items]**                                                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                             | $ 2,187                              |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                                                                        | 4,770                                |                  |                  |                  |                  |
| Commercial Auto Line | Commercial Insurance                                                                                              |                                 |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                             | 1,768                                |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                      | 1,264                                |                  |                  |                  |                  |
| All outstanding liabilities before, net of reinsurance                                                                                   | 12                                   |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                                                                        | 516                                  |                  |                  |                  |                  |
| 2012 | Commercial Auto Line | Commercial Insurance                                                                                       |                                 |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                             | 294                                  | $ 288                 | $ 285                 | $ 273                 | $ 275                 |
| Total of incurred but not reported liabilities plus expected development on reported losses                                              | $ 2                                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                    | 43                                   |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                      | $ 278                                | 258                   | 222                   | 184                   | $ 129                 |
| 2013 | Commercial Auto Line | Commercial Insurance                                                                                       |                                 |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                             | 322                                  | 315                   | 305                   | 292                   |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                              | $ 9                                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                    | 46                                   |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                      | $ 285                                | 246                   | 199                   | $ 141                 |                  |
| 2014 | Commercial Auto Line | Commercial Insurance                                                                                       |                                 |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                             | 351                                  | 346                   | 333                   |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                              | $ 14                                 |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                    | 51                                   |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                      | $ 273                                | 223                   | $ 159                 |                  |                  |
| 2015 | Commercial Auto Line | Commercial Insurance                                                                                       |                                 |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                             | 384                                  | 374                   |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                              | $ 38                                 |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                    | 51                                   |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                      | $ 244                                | $ 173                 |                  |                  |                  |
| 2016 | Commercial Auto Line | Commercial Insurance                                                                                       |                                 |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                             | 417                                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                              | $ 81                                 |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                    | 49                                   |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                      | $ 184                                |                  |                  |                  |                  |

| 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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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after reinsurance, of paid claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of incurred-but-not-reported (IBNR) liabilities plus expected development on reported claims, for claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of incurred claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts for accident years not separately presented in claim development information. Excludes unallocated claim adjustment expense.
+ References
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| - Definition
Number of reported claims for short-duration insurance contracts.
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