| **Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions** | Mar. 31, 2019 | Dec. 31, 2018 |
|----------------------------------------------------------------------------------------------------|---------------|---------------|
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | $ 2,478       | $ 12,689      |
| Gross Unrealized Losses, Less than 12 Months                                                       | 53            | 404           |
| Estimated Fair Value, 12 Months or Longer                                                          | 3,612         | 2,614         |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 87            | 117           |
| Estimated Fair Value, Total                                                                        | 6,090         | 15,303        |
| Gross Unrealized Losses, Total                                                                     | 140           | 521           |
| Corporate and other bonds                                                                          |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 1,656         | 8,543         |
| Gross Unrealized Losses, Less than 12 Months                                                       | 39            | 340           |
| Estimated Fair Value, 12 Months or Longer                                                          | 1,743         | 825           |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 60            | 55            |
| Estimated Fair Value, Total                                                                        | 3,399         | 9,368         |
| Gross Unrealized Losses, Total                                                                     | 99            | 395           |
| States, municipalities and political subdivisions                                                  |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 14            | 517           |
| Gross Unrealized Losses, Less than 12 Months                                                       | 0             | 8             |
| Estimated Fair Value, 12 Months or Longer                                                          | 3             | 5             |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 0             | 1             |
| Estimated Fair Value, Total                                                                        | 17            | 522           |
| Gross Unrealized Losses, Total                                                                     |          | 9             |
| Residential mortgage-backed                                                                        |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 94            | 1,932         |
| Gross Unrealized Losses, Less than 12 Months                                                       | 0             | 23            |
| Estimated Fair Value, 12 Months or Longer                                                          | 1,494         | 1,119         |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 20            | 34            |
| Estimated Fair Value, Total                                                                        | 1,588         | 3,051         |
| Gross Unrealized Losses, Total                                                                     | 20            | 57            |
| Commercial mortgage-backed                                                                         |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 184           | 728           |
| Gross Unrealized Losses, Less than 12 Months                                                       | 2             | 10            |
| Estimated Fair Value, 12 Months or Longer                                                          | 236           | 397           |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 5             | 22            |
| Estimated Fair Value, Total                                                                        | 420           | 1,125         |
| Gross Unrealized Losses, Total                                                                     | 7             | 32            |
| Other asset-backed                                                                                 |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 450           | 834           |
| Gross Unrealized Losses, Less than 12 Months                                                       | 10            | 21            |
| Estimated Fair Value, 12 Months or Longer                                                          | 95            | 125           |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 2             | 3             |
| Estimated Fair Value, Total                                                                        | 545           | 959           |
| Gross Unrealized Losses, Total                                                                     | 12            | 24            |
| Asset-backed                                                                                       |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 728           | 3,494         |
| Gross Unrealized Losses, Less than 12 Months                                                       | 12            | 54            |
| Estimated Fair Value, 12 Months or Longer                                                          | 1,825         | 1,641         |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 27            | 59            |
| Estimated Fair Value, Total                                                                        | 2,553         | 5,135         |
| Gross Unrealized Losses, Total                                                                     | 39            | 113           |
| U.S. Treasury and obligations of government-sponsored enterprises                                  |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 48            | 21            |
| Gross Unrealized Losses, Less than 12 Months                                                       | 1             | 0             |
| Estimated Fair Value, 12 Months or Longer                                                          | 14            | 19            |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 0             | 0             |
| Estimated Fair Value, Total                                                                        | 62            | 40            |
| Gross Unrealized Losses, Total                                                                     | 1             | 0             |
| Foreign government                                                                                 |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 months                                                          | 32            | 114           |
| Gross Unrealized Losses, Less than 12 Months                                                       | 1             | 2             |
| Estimated Fair Value, 12 Months or Longer                                                          | 27            | 124           |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 0             | 2             |
| Estimated Fair Value, Total                                                                        | 59            | 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.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563
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| 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 less than 12 months.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
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| 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 unrealized loss position.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
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| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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