| **Investments (Securities in a gross unrealized loss position) (Details) (USD $)In Millions, unless otherwise specified** | Mar. 31, 2013 | Dec. 31, 2012 |
|---------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Estimated Fair Value, Less Than 12 Months                                                                                 | $ 2,743       | $ 1,819       |
| Gross Unrealized Losses, Less than 12 Months                                                                              | 51            | 27            |
| Estimated Fair Value, 12 Months or Longer                                                                                 | 659           | 866           |
| Gross Unrealized Losses, 12 Months or Longer                                                                              | 107           | 128           |
| Estimated Fair Value, Total                                                                                               | 3,402         | 2,685         |
| Gross Unrealized Losses, Total                                                                                            | 158           | 155           |
| Corporate and other bonds [Member]                                                                                        |          |          |
| Estimated Fair Value, Less Than 12 Months                                                                                 | 951           | 846           |
| Gross Unrealized Losses, Less than 12 Months                                                                              | 13            | 13            |
| Estimated Fair Value, 12 Months or Longer                                                                                 | 50            | 108           |
| Gross Unrealized Losses, 12 Months or Longer                                                                              | 5             | 8             |
| Estimated Fair Value, Total                                                                                               | 1,001         | 954           |
| Gross Unrealized Losses, Total                                                                                            | 18            | 21            |
| States, municipalities and political subdivisions [Member]                                                                |          |          |
| Estimated Fair Value, Less Than 12 Months                                                                                 | 683           | 254           |
| Gross Unrealized Losses, Less than 12 Months                                                                              | 16            | 5             |
| Estimated Fair Value, 12 Months or Longer                                                                                 | 121           | 165           |
| Gross Unrealized Losses, 12 Months or Longer                                                                              | 43            | 39            |
| Estimated Fair Value, Total                                                                                               | 804           | 419           |
| Gross Unrealized Losses, Total                                                                                            | 59            | 44            |
| Residential mortgage-backed [Member]                                                                                      |          |          |
| Estimated Fair Value, Less Than 12 Months                                                                                 | 920           | 583           |
| Gross Unrealized Losses, Less than 12 Months                                                                              | 19            | 5             |
| Estimated Fair Value, 12 Months or Longer                                                                                 | 347           | 452           |
| Gross Unrealized Losses, 12 Months or Longer                                                                              | 51            | 66            |
| Estimated Fair Value, Total                                                                                               | 1,267         | 1,035         |
| Gross Unrealized Losses, Total                                                                                            | 70            | 71            |
| Commercial mortgage-backed [Member]                                                                                       |          |          |
| Estimated Fair Value, Less Than 12 Months                                                                                 | 155           | 85            |
| Gross Unrealized Losses, Less than 12 Months                                                                              | 2             | 2             |
| Estimated Fair Value, 12 Months or Longer                                                                                 | 141           | 141           |
| Gross Unrealized Losses, 12 Months or Longer                                                                              | 8             | 15            |
| Estimated Fair Value, Total                                                                                               | 296           | 226           |
| Gross Unrealized Losses, Total                                                                                            | 10            | 17            |
| Total asset-backed [Member]                                                                                               |          |          |
| Estimated Fair Value, Less Than 12 Months                                                                                 | 1,075         | 668           |
| Gross Unrealized Losses, Less than 12 Months                                                                              | 21            | 7             |
| Estimated Fair Value, 12 Months or Longer                                                                                 | 488           | 593           |
| Gross Unrealized Losses, 12 Months or Longer                                                                              | 59            | 81            |
| Estimated Fair Value, Total                                                                                               | 1,563         | 1,261         |
| Gross Unrealized Losses, Total                                                                                            | 80            | 88            |
| U.S. Treasury and obligations of government-sponsored enterprises [Member]                                                |          |          |
| Estimated Fair Value, Less Than 12 Months                                                                                 |          | 23            |
| Gross Unrealized Losses, Less than 12 Months                                                                              |          | 1             |
| Estimated Fair Value, 12 Months or Longer                                                                                 |          |     |
| Gross Unrealized Losses, 12 Months or Longer                                                                              |          |     |
| Estimated Fair Value, Total                                                                                               |          | 23            |
| Gross Unrealized Losses, Total                                                                                            |          | 1             |
| Redeemable preferred stock [Member]                                                                                       |          |          |
| Estimated Fair Value, Less Than 12 Months                                                                                 | 34            | 28            |
| Gross Unrealized Losses, Less than 12 Months                                                                              | 1             | 1             |
| Estimated Fair Value, 12 Months or Longer                                                                                 |     |     |
| Gross Unrealized Losses, 12 Months or Longer                                                                              |     |     |
| Estimated Fair Value, Total                                                                                               | 34            | 28            |
| Gross Unrealized Losses, Total                                                                                            | $ 1           | $ 1           |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for securities which are categorized neither as held-to-maturity nor trading securities.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of the excess of amortized cost basis over fair value of securities in a loss position and categorized neither as held-to-maturity nor trading securities.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for less than twelve months for securities categorized neither as held-to-maturity nor trading securities.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Emerging Issues Task Force (EITF) -Number 03-1 -Paragraph 21 -Subparagraph a(2) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27337-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27290-111563Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS115-1/124-1 -Paragraph 17 -Subparagraph a(1) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
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