| **Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions** | Sep. 30, 2020 | Dec. 31, 2019 |
|----------------------------------------------------------------------------------------------------|---------------|---------------|
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | $ 3,001       | $ 2,148       |
| Gross Unrealized Losses, Less than 12 Months                                                       | 176           | 31            |
| Estimated Fair Value, 12 Months or Longer                                                          | 101           | 272           |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 5             | 14            |
| Estimated Fair Value, Total                                                                        | 3,102         | 2,420         |
| Gross Unrealized Losses, Total                                                                     | 181           | 45            |
| Corporate and other bonds                                                                          |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 1,579         | 914           |
| Gross Unrealized Losses, Less than 12 Months                                                       | 86            | 21            |
| Estimated Fair Value, 12 Months or Longer                                                          | 56            | 186           |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 3             | 11            |
| Estimated Fair Value, Total                                                                        | 1,635         | 1,100         |
| Gross Unrealized Losses, Total                                                                     | 89            | 32            |
| States, municipalities and political subdivisions                                                  |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 154           | 34            |
| Gross Unrealized Losses, Less than 12 Months                                                       | 1             | 0             |
| Estimated Fair Value, 12 Months or Longer                                                          | 0             | 0             |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 0             | 0             |
| Estimated Fair Value, Total                                                                        | 154           | 34            |
| Gross Unrealized Losses, Total                                                                     | 1             |          |
| Residential mortgage-backed                                                                        |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 98            | 249           |
| Gross Unrealized Losses, Less than 12 Months                                                       | 1             | 1             |
| Estimated Fair Value, 12 Months or Longer                                                          | 13            | 30            |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 0             | 0             |
| Estimated Fair Value, Total                                                                        | 111           | 279           |
| Gross Unrealized Losses, Total                                                                     | 1             | 1             |
| Commercial mortgage-backed                                                                         |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 693           | 381           |
| Gross Unrealized Losses, Less than 12 Months                                                       | 69            | 3             |
| Estimated Fair Value, 12 Months or Longer                                                          | 19            | 20            |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 1             | 2             |
| Estimated Fair Value, Total                                                                        | 712           | 401           |
| Gross Unrealized Losses, Total                                                                     | 70            | 5             |
| Other asset-backed                                                                                 |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 432           | 449           |
| Gross Unrealized Losses, Less than 12 Months                                                       | 18            | 3             |
| Estimated Fair Value, 12 Months or Longer                                                          | 13            | 33            |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 1             | 1             |
| Estimated Fair Value, Total                                                                        | 445           | 482           |
| Gross Unrealized Losses, Total                                                                     | 19            | 4             |
| Asset-backed                                                                                       |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 1,223         | 1,079         |
| Gross Unrealized Losses, Less than 12 Months                                                       | 88            | 7             |
| Estimated Fair Value, 12 Months or Longer                                                          | 45            | 83            |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 2             | 3             |
| Estimated Fair Value, Total                                                                        | 1,268         | 1,162         |
| Gross Unrealized Losses, Total                                                                     | 90            | 10            |
| U.S. Treasury and obligations of government-sponsored enterprises                                  |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 25            | 62            |
| Gross Unrealized Losses, Less than 12 Months                                                       | 1             | 2             |
| Estimated Fair Value, 12 Months or Longer                                                          | 0             | 2             |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 0             | 0             |
| Estimated Fair Value, Total                                                                        | 25            | 64            |
| Gross Unrealized Losses, Total                                                                     | 1             | 2             |
| Foreign government                                                                                 |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 20            | 59            |
| Gross Unrealized Losses, Less than 12 Months                                                       | 0             | 1             |
| Estimated Fair Value, 12 Months or Longer                                                          | 0             | 1             |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 0             | 0             |
| Estimated Fair Value, Total                                                                        | 20            | 60            |
| Gross Unrealized Losses, Total                                                                     | $ 0           | $ 1           |

| 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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