| **Fair Value (Assets and liabilities measured at fair value on a recurring basis) (Details) - USD ($) $ in Millions** | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 |
|-----------------------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | $ 42,207      | $ 39,546      |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 865           | 780           |          |
| Short term and other                                                                                                  | 1,741         | 1,165         |          |
| Total assets                                                                                                          | 44,813        | 41,491        |               |
| Other liabilities                                                                                                     | 7             | (4)           |          |
| Total liabilities                                                                                                     | 7             | (4)           |          |
| Corporate and other bonds                                                                                             |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 22,728        | 19,814        |          |
| States, municipalities and political subdivisions                                                                     |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 10,652        | 10,748        |          |
| Asset-backed                                                                                                          |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 8,827         | 8,984         |          |
| Common Stock                                                                                                          |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 142           | 148           |          |
| Preferred stock                                                                                                       |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 723           | 632           |          |
| Level 1                                                                                                               |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 175           | 196           |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 189           | 192           |          |
| Short term and other                                                                                                  | 397           | 216           |          |
| Total assets                                                                                                          | 761           | 604           |          |
| Other liabilities                                                                                                     | 0             | 0             |          |
| Total liabilities                                                                                                     | 0             | 0             |          |
| Level 1 | Corporate and other bonds                                                                                   |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 175           | 196           |          |
| Level 1 | States, municipalities and political subdivisions                                                           |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 0             | 0             |          |
| Level 1 | Asset-backed                                                                                                |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 0             | 0             |          |
| Level 1 | Common Stock                                                                                                |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 135           | 144           |          |
| Level 1 | Preferred stock                                                                                             |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 54            | 48            |          |
| Level 2                                                                                                               |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 41,399        | 38,931        |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 658           | 570           |          |
| Short term and other                                                                                                  | 1,344         | 949           |          |
| Total assets                                                                                                          | 43,401        | 40,450        |          |
| Other liabilities                                                                                                     | 7             | (4)           |          |
| Total liabilities                                                                                                     | 7             | (4)           |          |
| Level 2 | Corporate and other bonds                                                                                   |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 22,085        | 19,396        |          |
| Level 2 | States, municipalities and political subdivisions                                                           |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 10,652        | 10,748        |          |
| Level 2 | Asset-backed                                                                                                |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 8,662         | 8,787         |          |
| Level 2 | Common Stock                                                                                                |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 0             | 0             |          |
| Level 2 | Preferred stock                                                                                             |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 658           | 570           |          |
| Level 3                                                                                                               |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 633           | 419           |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 18            | 18            |          |
| Short term and other                                                                                                  | 0             | 0             |          |
| Total assets                                                                                                          | 651           | 437           |          |
| Other liabilities                                                                                                     | 0             | 0             |          |
| Total liabilities                                                                                                     | 0             | 0             |          |
| Level 3 | Corporate and other bonds                                                                                   |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 468           | 222           |          |
| Level 3 | States, municipalities and political subdivisions                                                           |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 0             | 0             |          |
| Level 3 | Asset-backed                                                                                                |          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,126 and $38,085)                                       | 165           | 197           |          |
| Level 3 | Common Stock                                                                                                |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | 7             | 4             |          |
| Level 3 | Preferred stock                                                                                             |          |          |          |
| **Equity securities:**                                                                                                |          |          |          |
| Equity securities                                                                                                     | $ 11          | $ 14          |          |

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Short-term Investments and Other
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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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| - Definition
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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