| **Property Casualty Loss And Loss Expenses (Claims Development - Excess and Surplus Lines) (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 ($) | Dec. 31, 2011 USD ($) |
|---------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|
| **Claims Development [Line Items]**                                                                                                               |                                 |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                                      | $ 2,187                              |                  |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                                                                                 | 4,770                                |                  |                  |                  |                  |                  |
| Excess and Surplus Lines Insurance                                                                                                                |                                 |                  |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                               |                                 |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                                      | 366                                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                               | 144                                  |                  |                  |                  |                  |                  |
| All outstanding liabilities before, net of reinsurance                                                                                            | 1                                    |                  |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                                                                                 | 223                                  |                  |                  |                  |                  |                  |
| 2011 | Excess and Surplus Lines Insurance                                                                                                         |                                 |                  |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                               |                                 |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                                      | 35                                   | $ 36                  | $ 38                  | $ 44                  | $ 47                  | $ 48                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                                       | $ 0                                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                             | 1                                    |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                               | $ 32                                 | 30                    | 27                    | 23                    | 14                    | $ 8                   |
| 2012 | Excess and Surplus Lines Insurance                                                                                                         |                                 |                  |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                               |                                 |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                                      | 37                                   | 40                    | 49                    | 56                    | 67                    |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                                       | $ 2                                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                             | 1                                    |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                               | $ 29                                 | 25                    | 19                    | 15                    | $ 9                   |                  |
| 2013 | Excess and Surplus Lines Insurance                                                                                                         |                                 |                  |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                               |                                 |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                                      | 45                                   | 54                    | 64                    | 74                    |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                                       | $ 9                                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                             | 2                                    |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                               | $ 27                                 | 20                    | 12                    | $ 7                   |                  |                  |
| 2014 | Excess and Surplus Lines Insurance                                                                                                         |                                 |                  |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                               |                                 |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                                      | 75                                   | 82                    | 95                    |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                                       | $ 26                                 |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                             | 2                                    |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                               | $ 27                                 | 17                    | $ 9                   |                  |                  |                  |
| 2015 | Excess and Surplus Lines Insurance                                                                                                         |                                 |                  |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                               |                                 |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                                      | 81                                   | 96                    |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                                       | $ 40                                 |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                             | 2                                    |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                               | $ 19                                 | $ 8                   |                  |                  |                  |                  |
| 2016 | Excess and Surplus Lines Insurance                                                                                                         |                                 |                  |                  |                  |                  |                  |
| **Claims Development [Line Items]**                                                                                                               |                                 |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance                                                                                                      | 93                                   |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses                                                       | $ 54                                 |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                                                             | 2                                    |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                                                               | $ 10                                 |                  |                  |                  |                  |                  |

| 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 reinsurance, of paid claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671311-158438
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4D -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671317-158438Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4E -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671321-158438
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671311-158438
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of reported claims for short-duration insurance contracts.
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