| **Fair Value (Assets and liabilities measured at fair value on a recurring basis) (Details) - USD ($) $ in Millions** | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 |
|-----------------------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | $ 39,546      | $ 41,487      |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 780           | 695           |          |
| Estimated Fair Value                                                                                                  |          | 695           |          |
| Short term and other                                                                                                  | 1,165         | 1,354         |          |
| Total assets                                                                                                          | 41,491        | 43,536        |               |
| Other liabilities                                                                                                     | (4)           | 3             |          |
| Total liabilities                                                                                                     | (4)           | 3             |          |
| Corporate and other bonds                                                                                             |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 19,814        | 19,374        |          |
| States, municipalities and political subdivisions                                                                     |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 10,748        | 14,027        |          |
| Asset-backed                                                                                                          |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 8,984         | 8,086         |          |
| Common Stock                                                                                                          |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 148           |          |          |
| Estimated Fair Value                                                                                                  |          | 27            |          |
| Preferred stock                                                                                                       |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 632           |          |          |
| Estimated Fair Value                                                                                                  |          | 668           |          |
| Level 1                                                                                                               |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 196           | 128           |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 192           |          |          |
| Estimated Fair Value                                                                                                  |          | 91            |          |
| Short term and other                                                                                                  | 216           | 396           |          |
| Total assets                                                                                                          | 604           | 615           |          |
| Other liabilities                                                                                                     | 0             | 0             |          |
| Total liabilities                                                                                                     | 0             | 0             |          |
| Level 1 | Corporate and other bonds                                                                                   |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 196           | 128           |          |
| Level 1 | States, municipalities and political subdivisions                                                           |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 0             | 0             |          |
| Level 1 | Asset-backed                                                                                                |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 0             | 0             |          |
| Level 1 | Common Stock                                                                                                |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 144           |          |          |
| Estimated Fair Value                                                                                                  |          | 23            |          |
| Level 1 | Preferred stock                                                                                             |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 48            |          |          |
| Estimated Fair Value                                                                                                  |          | 68            |          |
| Level 2                                                                                                               |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 38,931        | 40,925        |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 570           |          |          |
| Estimated Fair Value                                                                                                  |          | 584           |          |
| Short term and other                                                                                                  | 949           | 958           |          |
| Total assets                                                                                                          | 40,450        | 42,467        |          |
| Other liabilities                                                                                                     | (4)           | 3             |          |
| Total liabilities                                                                                                     | (4)           | 3             |          |
| Level 2 | Corporate and other bonds                                                                                   |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 19,396        | 19,148        |          |
| Level 2 | States, municipalities and political subdivisions                                                           |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 10,748        | 14,026        |          |
| Level 2 | Asset-backed                                                                                                |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 8,787         | 7,751         |          |
| Level 2 | Common Stock                                                                                                |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 0             |          |          |
| Estimated Fair Value                                                                                                  |          | 0             |          |
| Level 2 | Preferred stock                                                                                             |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 570           |          |          |
| Estimated Fair Value                                                                                                  |          | 584           |          |
| Level 3                                                                                                               |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 419           | 434           |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 18            |          |          |
| Estimated Fair Value                                                                                                  |          | 20            |          |
| Short term and other                                                                                                  | 0             | 0             |          |
| Total assets                                                                                                          | 437           | 454           |          |
| Other liabilities                                                                                                     | 0             | 0             |          |
| Total liabilities                                                                                                     | 0             | 0             |          |
| Level 3 | Corporate and other bonds                                                                                   |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 222           | 98            |          |
| Level 3 | States, municipalities and political subdivisions                                                           |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 0             | 1             |          |
| Level 3 | Asset-backed                                                                                                |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Debt securities                                                                                                       | 197           | 335           |          |
| Level 3 | Common Stock                                                                                                |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 4             |          |          |
| Estimated Fair Value                                                                                                  |          | 4             |          |
| Level 3 | Preferred stock                                                                                             |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | $ 14          |          |          |
| Estimated Fair Value                                                                                                  |          | $ 16          |          |

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
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Amount of equity securities categorized neither as held-to-maturity nor as trading.
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Amount of investment in debt security measured at amortized cost (held-to-maturity), investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and investment in debt security measured at fair value with change in fair value recognized in net income (trading).
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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).
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Fair value of financial and nonfinancial obligations.
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Fair value portion of other liabilities.
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