| **Investments (Summary of fixed maturity and equity securities) (Details) - USD ($) $ in Millions** | Mar. 31, 2019 | Dec. 31, 2018 |
|-----------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fixed maturity securities available-for-sale:**                                                   |          |          |
| Cost or Amortized Cost                                                                              | $ 37,912      | $ 38,081      |
| Gross Unrealized Gains                                                                              | 2,753         | 1,982         |
| Gross Unrealized Losses                                                                             | 140           | 521           |
| Estimated Fair Value                                                                                | 40,525        | 39,542        |
| **Total fixed maturity securities trading**                                                         |          |          |
| Debt securities, amortized cost                                                                     | 37,940        | 38,085        |
| Debt securities                                                                                     | 40,553        | 39,546        |
| Corporate and other bonds                                                                           |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |
| Cost or Amortized Cost                                                                              | 19,296        | 18,764        |
| Gross Unrealized Gains                                                                              | 1,269         | 791           |
| Gross Unrealized Losses                                                                             | 99            | 395           |
| Estimated Fair Value                                                                                | 20,466        | 19,160        |
| **Total fixed maturity securities trading**                                                         |          |          |
| Debt securities                                                                                     | 21,163        | 19,814        |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| States, municipalities and political subdivisions                                                   |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |
| Cost or Amortized Cost                                                                              | 9,279         | 9,681         |
| Gross Unrealized Gains                                                                              | 1,299         | 1,076         |
| Gross Unrealized Losses                                                                             | 0             | 9             |
| Estimated Fair Value                                                                                | 10,578        | 10,748        |
| **Total fixed maturity securities trading**                                                         |          |          |
| Debt securities                                                                                     | 10,596        | 10,748        |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Residential mortgage-backed                                                                         |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |
| Cost or Amortized Cost                                                                              | 4,760         | 4,815         |
| Gross Unrealized Gains                                                                              | 92            | 68            |
| Gross Unrealized Losses                                                                             | 20            | 57            |
| Estimated Fair Value                                                                                | 4,832         | 4,826         |
| **Total fixed maturity securities trading**                                                         |          |          |
| Unrealized OTTI Losses (Gains)                                                                      | (22)          | (20)          |
| Commercial mortgage-backed                                                                          |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |
| Cost or Amortized Cost                                                                              | 2,026         | 2,200         |
| Gross Unrealized Gains                                                                              | 53            | 28            |
| Gross Unrealized Losses                                                                             | 7             | 32            |
| Estimated Fair Value                                                                                | 2,072         | 2,196         |
| **Total fixed maturity securities trading**                                                         |          |          |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Other asset-backed                                                                                  |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |
| Cost or Amortized Cost                                                                              | 1,877         | 1,975         |
| Gross Unrealized Gains                                                                              | 25            | 11            |
| Gross Unrealized Losses                                                                             | 12            | 24            |
| Estimated Fair Value                                                                                | 1,890         | 1,962         |
| **Total fixed maturity securities trading**                                                         |          |          |
| Unrealized OTTI Losses (Gains)                                                                      | (3)           | 0             |
| Asset-backed                                                                                        |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |
| Cost or Amortized Cost                                                                              | 8,663         | 8,990         |
| Gross Unrealized Gains                                                                              | 170           | 107           |
| Gross Unrealized Losses                                                                             | 39            | 113           |
| Estimated Fair Value                                                                                | 8,794         | 8,984         |
| **Total fixed maturity securities trading**                                                         |          |          |
| Debt securities                                                                                     | 8,794         | 8,984         |
| Unrealized OTTI Losses (Gains)                                                                      | (25)          | (20)          |
| U.S. Treasury and obligations of government-sponsored enterprises                                   |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |
| Cost or Amortized Cost                                                                              | 162           | 156           |
| Gross Unrealized Gains                                                                              | 3             | 3             |
| Gross Unrealized Losses                                                                             | 1             | 0             |
| Estimated Fair Value                                                                                | 164           | 159           |
| **Total fixed maturity securities trading**                                                         |          |          |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Foreign government                                                                                  |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |
| Cost or Amortized Cost                                                                              | 502           | 480           |
| Gross Unrealized Gains                                                                              | 12            | 5             |
| Gross Unrealized Losses                                                                             | 1             | 4             |
| Estimated Fair Value                                                                                | 513           | 481           |
| **Total fixed maturity securities trading**                                                         |          |          |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Redeemable preferred stock                                                                          |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |
| Cost or Amortized Cost                                                                              | 10            | 10            |
| Gross Unrealized Gains                                                                              | 0             | 0             |
| Gross Unrealized Losses                                                                             | 0             | 0             |
| Estimated Fair Value                                                                                | 10            | 10            |
| **Total fixed maturity securities trading**                                                         |          |          |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Fixed maturity securities                                                                           |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |
| Cost or Amortized Cost                                                                              | 37,912        | 38,081        |
| Gross Unrealized Gains                                                                              | 2,753         | 1,982         |
| Gross Unrealized Losses                                                                             | 140           | 521           |
| Estimated Fair Value                                                                                | 40,525        | 39,542        |
| **Total fixed maturity securities trading**                                                         |          |          |
| Unrealized OTTI Losses (Gains)                                                                      | (25)          | (20)          |
| Total fixed maturity securities trading                                                             |          |          |
| **Total fixed maturity securities trading**                                                         |          |          |
| Cost or amortized cost, trading securities                                                          | 28            | 4             |
| Estimated fair value, trading securities                                                            | $ 28          | $ 4           |

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