| **Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Incurred Claims and Allocated Claim Adjustment Expenses Commercial - General Liability (Details) - Commercial - General Liability [Member] $ in Millions** | 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 ($) | Dec. 31, 2007 USD ($) |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                              | $ 1,382               |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                         | $ 6,065               |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2007 [Member]                                                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                             | 53,553                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                              | $ 28                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                         | $ 540                 | $ 548                 | $ 545                 | $ 559                 | $ 600                 | $ 610                 | $ 639                 | $ 678                 | $ 673                 | $ 774                 |
| Short-duration Insurance Contracts, Accident Year 2008 [Member]                                                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                             | 44,586                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                              | $ 18                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                         | $ 591                 | 600                   | 613                   | 632                   | 633                   | 647                   | 630                   | 604                   | $ 611                 |                  |
| Short-duration Insurance Contracts, Accident Year 2009 [Member]                                                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                             | 43,955                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                              | $ 16                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                         | $ 622                 | 619                   | 623                   | 629                   | 633                   | 634                   | 637                   | $ 591                 |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2010 [Member]                                                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                             | 43,378                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                              | $ 25                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                         | $ 659                 | 675                   | 695                   | 649                   | 599                   | 597                   | $ 566                 |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2011 [Member]                                                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                             | 38,101                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                              | $ 41                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                         | $ 621                 | 611                   | 610                   | 564                   | 534                   | $ 537                 |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2012 [Member]                                                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                             | 34,037                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                              | $ 63                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                         | $ 558                 | 570                   | 579                   | 563                   | $ 539                 |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2013 [Member]                                                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                             | 32,897                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                              | $ 142                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                         | $ 643                 | 634                   | 645                   | $ 615                 |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2014 [Member]                                                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                             | 26,744                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                              | $ 224                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                         | $ 635                 | 634                   | $ 627                 |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2015 [Member]                                                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                             | 21,687                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                              | $ 330                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                         | $ 574                 | $ 573                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Accident Year 2016 [Member]                                                                                                                                                                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Number of Reported Claims per Million of Net Earned Premium                                                                                                                                                                             | 16,720                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                                              | $ 495                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                                                         | $ 622                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |

| X                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
[Line Items] for Net Incurred Claim and Claim Adjustment Expense [Table]
+ References
No definition available.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Reported Claims per Million of Net Earned Premium
+ References
No definition available.
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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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