| **Fair Value (Assets and liabilities measured at fair value on a recurring basis) (Details) - USD ($) $ in Millions**         | Sep. 30, 2020 | Dec. 31, 2019 |
|-------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | $ 43,901      | $ 42,207      |
| **Equity securities:**                                                                                                        |          |          |
| Equity securities                                                                                                             | 919           | 865           |
| Short term and other                                                                                                          | 1,326         | 1,741         |
| Total assets                                                                                                                  | 46,146        | 44,813        |
| Other liabilities                                                                                                             | 16            | 7             |
| Total liabilities                                                                                                             | 16            | 7             |
| Corporate and other bonds                                                                                                     |          |          |
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 24,525        | 22,728        |
| States, municipalities and political subdivisions                                                                             |          |          |
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 11,224        | 10,652        |
| Asset-backed                                                                                                                  |          |          |
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 8,152         | 8,827         |
| Common Stock                                                                                                                  |          |          |
| **Equity securities:**                                                                                                        |          |          |
| Equity securities                                                                                                             | 173           | 142           |
| Non-redeemable preferred stock                                                                                                |          |          |
| **Equity securities:**                                                                                                        |          |          |
| Equity securities                                                                                                             | 746           | 723           |
| Level 1                                                                                                                       |          |          |
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 372           | 175           |
| **Equity securities:**                                                                                                        |          |          |
| Equity securities                                                                                                             | 222           | 189           |
| Short term and other                                                                                                          | 1,301         | 397           |
| Total assets                                                                                                                  | 1,895         | 761           |
| 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 $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 372           | 175           |
| Level 1 | States, municipalities and political subdivisions                                                                   |          |          |
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 0             | 0             |
| Level 1 | Asset-backed                                                                                                        |          |          |
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 0             | 0             |
| Level 1 | Common Stock                                                                                                        |          |          |
| **Equity securities:**                                                                                                        |          |          |
| Equity securities                                                                                                             | 157           | 135           |
| Level 1 | Non-redeemable preferred stock                                                                                      |          |          |
| **Equity securities:**                                                                                                        |          |          |
| Equity securities                                                                                                             | 65            | 54            |
| Level 2                                                                                                                       |          |          |
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 42,555        | 41,399        |
| **Equity securities:**                                                                                                        |          |          |
| Equity securities                                                                                                             | 674           | 658           |
| Short term and other                                                                                                          | 25            | 1,344         |
| Total assets                                                                                                                  | 43,254        | 43,401        |
| Other liabilities                                                                                                             | 16            | 7             |
| Total liabilities                                                                                                             | 16            | 7             |
| Level 2 | Corporate and other bonds                                                                                           |          |          |
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 23,459        | 22,085        |
| Level 2 | States, municipalities and political subdivisions                                                                   |          |          |
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 11,179        | 10,652        |
| Level 2 | Asset-backed                                                                                                        |          |          |
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 7,917         | 8,662         |
| Level 2 | Common Stock                                                                                                        |          |          |
| **Equity securities:**                                                                                                        |          |          |
| Equity securities                                                                                                             | 0             | 0             |
| Level 2 | Non-redeemable preferred stock                                                                                      |          |          |
| **Equity securities:**                                                                                                        |          |          |
| Equity securities                                                                                                             | 674           | 658           |
| Level 3                                                                                                                       |          |          |
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 974           | 633           |
| **Equity securities:**                                                                                                        |          |          |
| Equity securities                                                                                                             | 23            | 18            |
| Short term and other                                                                                                          | 0             | 0             |
| Total assets                                                                                                                  | 997           | 651           |
| 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 $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 694           | 468           |
| Level 3 | States, municipalities and political subdivisions                                                                   |          |          |
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 45            | 0             |
| Level 3 | Asset-backed                                                                                                        |          |          |
| **Fixed maturity securities:**                                                                                                |          |          |
| Fixed maturity securities at fair value (amortized cost of $38,969 and $38,126, less allowance for credit loss of $47 and $-) | 235           | 165           |
| Level 3 | Common Stock                                                                                                        |          |          |
| **Equity securities:**                                                                                                        |          |          |
| Equity securities                                                                                                             | 16            | 7             |
| Level 3 | Non-redeemable preferred stock                                                                                      |          |          |
| **Equity securities:**                                                                                                        |          |          |
| Equity securities                                                                                                             | $ 7           | $ 11          |

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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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| - Definition
Fair value of financial and nonfinancial obligations.
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Fair value portion of other liabilities.
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