| **Claim and Claim Adjustment Expense Reserves (Commercial - General Liability - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - General Liability - Commercial $ in Millions** | Dec. 31, 2017 USD ($) claims | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2011 USD ($) | Dec. 31, 2010 USD ($) | Dec. 31, 2009 USD ($) | Dec. 31, 2008 USD ($) |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                   |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                             | $ 6,147                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                    | 1,412                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2008                                                                                                                                                                                                    |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                   |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                             | 592                          | $ 591                 | $ 600                 | $ 613                 | $ 632                 | $ 633                 | $ 647                 | $ 630                 | $ 604                 | $ 611                 |
| IBNR                                                                                                                                                                                                    | $ 13                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                    | 44,655                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2009                                                                                                                                                                                                    |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                   |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                             | $ 627                        | 622                   | 619                   | 623                   | 629                   | 633                   | 634                   | 637                   | $ 591                 |                  |
| IBNR                                                                                                                                                                                                    | $ 18                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                    | 44,038                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2010                                                                                                                                                                                                    |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                   |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                             | $ 654                        | 659                   | 675                   | 695                   | 649                   | 599                   | 597                   | $ 566                 |                  |                  |
| IBNR                                                                                                                                                                                                    | $ 19                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                    | 43,472                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2011                                                                                                                                                                                                    |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                   |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                             | $ 615                        | 621                   | 611                   | 610                   | 564                   | 534                   | $ 537                 |                  |                  |                  |
| IBNR                                                                                                                                                                                                    | $ 29                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                    | 38,216                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2012                                                                                                                                                                                                    |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                   |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                             | $ 569                        | 558                   | 570                   | 579                   | 563                   | $ 539                 |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                    | $ 34                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                    | 34,249                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2013                                                                                                                                                                                                    |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                   |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                             | $ 604                        | 643                   | 634                   | 645                   | $ 615                 |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                    | $ 62                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                    | 33,255                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2014                                                                                                                                                                                                    |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                   |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                             | $ 627                        | 635                   | 634                   | $ 627                 |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                    | $ 131                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                    | 27,478                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2015                                                                                                                                                                                                    |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                   |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                             | $ 585                        | 574                   | $ 573                 |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                    | $ 208                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                    | 23,082                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2016                                                                                                                                                                                                    |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                   |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                             | $ 647                        | $ 622                 |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                    | $ 351                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                    | 21,893                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2017                                                                                                                                                                                                    |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                   |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                             | $ 627                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                    | $ 547                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                    | 15,375                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |

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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
[Line Items] for Net Incurred Claim and Claim Adjustment Expense [Table]
+ 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
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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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of reported claims for short-duration insurance contracts.
+ References
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