| **Investments (Summary of fixed maturity and equity securities) (Details) - USD ($) $ in Millions** | Mar. 31, 2016 | Dec. 31, 2015 |
|-----------------------------------------------------------------------------------------------------|---------------|---------------|
| Cost or Amortized Cost                                                                              | $ 37,453      | $ 37,444      |
| Gross Unrealized Gain                                                                               | 3,275         | 2,717         |
| Gross Unrealized Loss                                                                               | 287           | 392           |
| Estimated fair value                                                                                | 40,441        | 39,769        |
| Corporate and other bonds [Member]                                                                  |          |          |
| Cost or Amortized Cost                                                                              | 17,076        | 17,080        |
| Gross Unrealized Gain                                                                               | 1,255         | 1,019         |
| Gross Unrealized Loss                                                                               | 224           | 342           |
| Estimated fair value                                                                                | 18,107        | 17,757        |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| States, municipalities and political subdivisions [Member]                                          |          |          |
| Cost or Amortized Cost                                                                              | 11,478        | 11,729        |
| Gross Unrealized Gain                                                                               | 1,704         | 1,453         |
| Gross Unrealized Loss                                                                               | 3             | 8             |
| Estimated fair value                                                                                | 13,179        | 13,174        |
| Unrealized OTTI Losses (Gains)                                                                      | (15)          | (4)           |
| Residential mortgage-backed [Member]                                                                |          |          |
| Cost or Amortized Cost                                                                              | 5,028         | 4,935         |
| Gross Unrealized Gain                                                                               | 197           | 154           |
| Gross Unrealized Loss                                                                               | 19            | 17            |
| Estimated fair value                                                                                | 5,206         | 5,072         |
| Unrealized OTTI Losses (Gains)                                                                      | (33)          | (37)          |
| Commercial mortgage-backed [Member]                                                                 |          |          |
| Cost or Amortized Cost                                                                              | 2,137         | 2,154         |
| Gross Unrealized Gain                                                                               | 78            | 55            |
| Gross Unrealized Loss                                                                               | 16            | 12            |
| Estimated fair value                                                                                | 2,199         | 2,197         |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Other asset-backed [Member]                                                                         |          |          |
| Cost or Amortized Cost                                                                              | 937           | 923           |
| Gross Unrealized Gain                                                                               | 4             | 6             |
| Gross Unrealized Loss                                                                               | 20            | 8             |
| Estimated fair value                                                                                | 921           | 921           |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Total asset-backed [Member]                                                                         |          |          |
| Cost or Amortized Cost                                                                              | 8,102         | 8,012         |
| Gross Unrealized Gain                                                                               | 279           | 215           |
| Gross Unrealized Loss                                                                               | 55            | 37            |
| Estimated fair value                                                                                | 8,326         | 8,190         |
| Unrealized OTTI Losses (Gains)                                                                      | (33)          | (37)          |
| U.S. Treasury and obligations of government-sponsored enterprises [Member]                          |          |          |
| Cost or Amortized Cost                                                                              | 133           | 62            |
| Gross Unrealized Gain                                                                               | 7             | 5             |
| Gross Unrealized Loss                                                                               | 0             | 0             |
| Estimated fair value                                                                                | 140           | 67            |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Foreign Government Debt Securities [Member]                                                         |          |          |
| Cost or Amortized Cost                                                                              | 447           | 334           |
| Gross Unrealized Gain                                                                               | 17            | 13            |
| Gross Unrealized Loss                                                                               | 1             | 1             |
| Estimated fair value                                                                                | 463           | 346           |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Redeemable preferred stock [Member]                                                                 |          |          |
| Cost or Amortized Cost                                                                              | 33            | 33            |
| Gross Unrealized Gain                                                                               | 2             | 2             |
| Gross Unrealized Loss                                                                               | 0             | 0             |
| Estimated fair value                                                                                | 35            | 35            |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Total fixed maturity securities available-for-sale [Member]                                         |          |          |
| Cost or Amortized Cost                                                                              | 37,269        | 37,250        |
| Gross Unrealized Gain                                                                               | 3,264         | 2,707         |
| Gross Unrealized Loss                                                                               | 283           | 388           |
| Estimated fair value                                                                                | 40,250        | 39,569        |
| Unrealized OTTI Losses (Gains)                                                                      | (48)          | (41)          |
| Total fixed maturity securities trading [Member]                                                    |          |          |
| Cost or Amortized Cost, Trading Securities                                                          | 2             | 3             |
| Estimated Fair Value, Trading Securities                                                            | 2             | 3             |
| Common stock [Member]                                                                               |          |          |
| Cost or Amortized Cost                                                                              | 37            | 46            |
| Gross Unrealized Gain                                                                               | 6             | 3             |
| Gross Unrealized Loss                                                                               | 2             | 1             |
| Estimated fair value                                                                                | 41            | 48            |
| Preferred stock [Member]                                                                            |          |          |
| Cost or Amortized Cost                                                                              | 145           | 145           |
| Gross Unrealized Gain                                                                               | 5             | 7             |
| Gross Unrealized Loss                                                                               | 2             | 3             |
| Estimated fair value                                                                                | 148           | 149           |
| Total equity securities available-for-sale [Member]                                                 |          |          |
| Cost or Amortized Cost                                                                              | 182           | 191           |
| Gross Unrealized Gain                                                                               | 11            | 10            |
| Gross Unrealized Loss                                                                               | 4             | 4             |
| Estimated fair value                                                                                | $ 189         | $ 197         |

| X                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The accumulated unrealized gain or loss position on Other Than Temporary Impariment (OTTI) Securities
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cost of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.
+ References
No definition available.
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