| **Fair Value (Assets and liabilities measured at fair value on a recurring basis) (Details) - USD ($) $ in Millions** | Jun. 30, 2019 | Dec. 31, 2018 |
|-----------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | $ 41,639      | $ 39,546      |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 830           | 780           |
| Short term and other                                                                                                  | 1,411         | 1,165         |
| Total assets                                                                                                          | 43,880        | 41,491        |
| **Liabilities**                                                                                                       |          |          |
| Other liabilities                                                                                                     | 8             | (4)           |
| Total liabilities                                                                                                     | 8             | (4)           |
| Corporate and other bonds                                                                                             |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | 22,120        | 19,814        |
| States, municipalities and political subdivisions                                                                     |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | 10,703        | 10,748        |
| Asset-backed                                                                                                          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | 8,816         | 8,984         |
| Common Stock                                                                                                          |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 135           | 148           |
| Preferred stock                                                                                                       |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 695           | 632           |
| Level 1                                                                                                               |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | 152           | 196           |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 180           | 192           |
| Short term and other                                                                                                  | 359           | 216           |
| Total assets                                                                                                          | 691           | 604           |
| **Liabilities**                                                                                                       |          |          |
| Other liabilities                                                                                                     | 0             | 0             |
| Total liabilities                                                                                                     | 0             | 0             |
| Level 1 | Corporate and other bonds                                                                                   |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | 152           | 196           |
| 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                                                                                                     | 129           | 144           |
| Level 1 | Preferred stock                                                                                             |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 51            | 48            |
| Level 2                                                                                                               |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | 40,956        | 38,931        |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 628           | 570           |
| Short term and other                                                                                                  | 1,052         | 949           |
| Total assets                                                                                                          | 42,636        | 40,450        |
| **Liabilities**                                                                                                       |          |          |
| Other liabilities                                                                                                     | 8             | (4)           |
| Total liabilities                                                                                                     | 8             | (4)           |
| Level 2 | Corporate and other bonds                                                                                   |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | 21,630        | 19,396        |
| Level 2 | States, municipalities and political subdivisions                                                           |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | 10,703        | 10,748        |
| Level 2 | Asset-backed                                                                                                |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | 8,623         | 8,787         |
| Level 2 | Common Stock                                                                                                |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 0             | 0             |
| Level 2 | Preferred stock                                                                                             |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 628           | 570           |
| Level 3                                                                                                               |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | 531           | 419           |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 22            | 18            |
| Short term and other                                                                                                  | 0             | 0             |
| Total assets                                                                                                          | 553           | 437           |
| **Liabilities**                                                                                                       |          |          |
| Other liabilities                                                                                                     | 0             | 0             |
| Total liabilities                                                                                                     | 0             | 0             |
| Level 3 | Corporate and other bonds                                                                                   |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | 338           | 222           |
| Level 3 | States, municipalities and political subdivisions                                                           |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | 0             | 0             |
| Level 3 | Asset-backed                                                                                                |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Debt securities                                                                                                       | 193           | 197           |
| Level 3 | Common Stock                                                                                                |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 6             | 4             |
| Level 3 | Preferred stock                                                                                             |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | $ 16          | $ 14          |

| X                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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Short-term Investments and Other
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| - Definition
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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| - Definition
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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| - Definition
Fair value of financial and nonfinancial obligations.
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Fair value portion of other liabilities.
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