| **Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Incurred Claims and Allocated Claim Adjustment Expenses Commercial - Auto (Details) - Commercial - Auto [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                                                                                                                    | $ 247                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                               | $ 2,630               |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 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                                                                                                                                                   | 67,473                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                    | $ 0                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                               | $ 352                 | $ 354                 | $ 355                 | $ 356                 | $ 358                 | $ 355                 | $ 360                 | $ 368                 | $ 367                 | $ 348                 |
| 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                                                                                                                                                   | 56,407                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                    | $ 0                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                               | $ 296                 | 298                   | 298                   | 305                   | 309                   | 306                   | 316                   | 323                   | $ 322                 |                  |
| 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                                                                                                                                                   | 47,325                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                    | $ 0                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                               | $ 272                 | 275                   | 277                   | 281                   | 278                   | 274                   | 272                   | $ 287                 |                  |                  |
| 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                                                                                                                                                   | 46,324                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                    | $ 1                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                               | $ 281                 | 286                   | 291                   | 283                   | 279                   | 274                   | $ 262                 |                  |                  |                  |
| 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                                                                                                                                                   | 46,676                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                    | $ 5                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                               | $ 285                 | 290                   | 293                   | 279                   | 273                   | $ 262                 |                  |                  |                  |                  |
| 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                                                                                                                                                   | 45,279                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                    | $ 11                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                               | $ 300                 | 296                   | 292                   | 282                   | $ 270                 |                  |                  |                  |                  |                  |
| 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                                                                                                                                                   | 38,513                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                    | $ 20                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                               | $ 241                 | 257                   | 259                   | $ 242                 |                  |                  |                  |                  |                  |                  |
| 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                                                                                                                                                   | 32,958                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                    | $ 40                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                               | $ 210                 | 221                   | $ 231                 |                  |                  |                  |                  |                  |                  |                  |
| 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                                                                                                                                                   | 29,714                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                    | $ 65                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                               | $ 197                 | $ 199                 |                  |                  |                  |                  |                  |                  |                  |                  |
| 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                                                                                                                                                   | 25,196                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net                                                                                                                    | $ 105                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net                                                                                                               | $ 196                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |

| 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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