| **Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions** | Dec. 31, 2019 | Dec. 31, 2018 |
|----------------------------------------------------------------------------------------------------|---------------|---------------|
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | $ 2,148       | $ 12,689      |
| Gross Unrealized Losses, Less than 12 Months                                                       | 31            | 404           |
| Estimated Fair Value, 12 Months or Longer                                                          | 272           | 2,614         |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 14            | 117           |
| Estimated Fair Value, Total                                                                        | 2,420         | 15,303        |
| Gross Unrealized Losses, Total                                                                     | 45            | 521           |
| Corporate and other bonds                                                                          |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 914           | 8,543         |
| Gross Unrealized Losses, Less than 12 Months                                                       | 21            | 340           |
| Estimated Fair Value, 12 Months or Longer                                                          | 186           | 825           |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 11            | 55            |
| Estimated Fair Value, Total                                                                        | 1,100         | 9,368         |
| Gross Unrealized Losses, Total                                                                     | 32            | 395           |
| States, municipalities and political subdivisions                                                  |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 34            | 517           |
| Gross Unrealized Losses, Less than 12 Months                                                       | 0             | 8             |
| Estimated Fair Value, 12 Months or Longer                                                          | 0             | 5             |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 0             | 1             |
| Estimated Fair Value, Total                                                                        | 34            | 522           |
| Gross Unrealized Losses, Total                                                                     |          | 9             |
| Residential mortgage-backed                                                                        |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 249           | 1,932         |
| Gross Unrealized Losses, Less than 12 Months                                                       | 1             | 23            |
| Estimated Fair Value, 12 Months or Longer                                                          | 30            | 1,119         |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 0             | 34            |
| Estimated Fair Value, Total                                                                        | 279           | 3,051         |
| Gross Unrealized Losses, Total                                                                     | 1             | 57            |
| Commercial mortgage-backed                                                                         |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 381           | 728           |
| Gross Unrealized Losses, Less than 12 Months                                                       | 3             | 10            |
| Estimated Fair Value, 12 Months or Longer                                                          | 20            | 397           |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 2             | 22            |
| Estimated Fair Value, Total                                                                        | 401           | 1,125         |
| Gross Unrealized Losses, Total                                                                     | 5             | 32            |
| Other asset-backed                                                                                 |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 449           | 834           |
| Gross Unrealized Losses, Less than 12 Months                                                       | 3             | 21            |
| Estimated Fair Value, 12 Months or Longer                                                          | 33            | 125           |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 1             | 3             |
| Estimated Fair Value, Total                                                                        | 482           | 959           |
| Gross Unrealized Losses, Total                                                                     | 4             | 24            |
| Asset-backed                                                                                       |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 1,079         | 3,494         |
| Gross Unrealized Losses, Less than 12 Months                                                       | 7             | 54            |
| Estimated Fair Value, 12 Months or Longer                                                          | 83            | 1,641         |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 3             | 59            |
| Estimated Fair Value, Total                                                                        | 1,162         | 5,135         |
| Gross Unrealized Losses, Total                                                                     | 10            | 113           |
| U.S. Treasury and obligations of government-sponsored enterprises                                  |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 62            | 21            |
| Gross Unrealized Losses, Less than 12 Months                                                       | 2             | 0             |
| Estimated Fair Value, 12 Months or Longer                                                          | 2             | 19            |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 0             | 0             |
| Estimated Fair Value, Total                                                                        | 64            | 40            |
| Gross Unrealized Losses, Total                                                                     | 2             | 0             |
| Foreign government                                                                                 |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 59            | 114           |
| Gross Unrealized Losses, Less than 12 Months                                                       | 1             | 2             |
| Estimated Fair Value, 12 Months or Longer                                                          | 1             | 124           |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 0             | 2             |
| Estimated Fair Value, Total                                                                        | 60            | 238           |
| Gross Unrealized Losses, Total                                                                     | $ 1           | $ 4           |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.
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| - Definition
Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.
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| - Definition
Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.
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| - Definition
Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.
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| - Definition
Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.
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No definition available.
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