| **Property Casualty Loss And Loss Expenses (Claims Development - Personal 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                                |                  |                  |                  |                  |
| Personal Auto | Personal Insurance                                                                                                     |                                 |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                    |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                           | 1,633                                |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                    | 1,388                                |                  |                  |                  |                  |
| All outstanding liabilities before, net of reinsurance                                                                                 | 4                                    |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                                                                      | 249                                  |                  |                  |                  |                  |
| 2012 | Personal Auto | Personal Insurance                                                                                              |                                 |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                    |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                           | 271                                  | $ 270                 | $ 269                 | $ 265                 | $ 260                 |
| Total of incurred but not reported liabilities plus expected development on reported losses                                            | $ 0                                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                  | 89                                   |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                    | $ 268                                | 262                   | 250                   | 223                   | $ 174                 |
| 2013 | Personal Auto | Personal Insurance                                                                                              |                                 |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                    |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                           | 298                                  | 293                   | 280                   | 280                   |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                            | $ 0                                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                  | 96                                   |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                    | $ 287                                | 272                   | 245                   | $ 190                 |                  |
| 2014 | Personal Auto | Personal Insurance                                                                                              |                                 |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                    |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                           | 325                                  | 317                   | 317                   |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                            | $ 0                                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                  | 104                                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                    | $ 298                                | 267                   | $ 210                 |                  |                  |
| 2015 | Personal Auto | Personal Insurance                                                                                              |                                 |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                    |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                           | 356                                  | 343                   |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                            | $ 12                                 |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                  | 108                                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                    | $ 292                                | $ 229                 |                  |                  |                  |
| 2016 | Personal Auto | Personal Insurance                                                                                              |                                 |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                    |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                           | 383                                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                            | $ 38                                 |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                  | 104                                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                    | $ 243                                |                  |                  |                  |                  |

| 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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