| **Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions** | Sep. 30, 2018 | Dec. 31, 2017 |
|----------------------------------------------------------------------------------------------------|---------------|---------------|
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 Months                                                          |          | $ 3,620       |
| Gross Unrealized Losses, Less than 12 Months                                                       |          | 40            |
| Estimated Fair Value, 12 Months or Longer                                                          |          | 1,455         |
| Gross Unrealized Losses, 12 Months or Longer                                                       |          | 49            |
| Estimated Fair Value, Total                                                                        |          | 5,075         |
| Gross Unrealized Losses, Total                                                                     |          | 89            |
| Corporate and other bonds                                                                          |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 Months                                                          | $ 7,546       |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 197           |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 342           |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 18            |          |
| Estimated Fair Value, Total                                                                        | 7,888         |          |
| Gross Unrealized Losses, Total                                                                     | 215           |          |
| Estimated Fair Value, Less than 12 Months                                                          |          | 1,354         |
| Gross Unrealized Losses, Less than 12 Months                                                       |          | 21            |
| Estimated Fair Value, 12 Months or Longer                                                          |          | 168           |
| Gross Unrealized Losses, 12 Months or Longer                                                       |          | 7             |
| Estimated Fair Value, Total                                                                        |          | 1,522         |
| Gross Unrealized Losses, Total                                                                     |          | 28            |
| States, municipalities and political subdivisions                                                  |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 Months                                                          | 807           |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 19            |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 2             |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 1             |          |
| Estimated Fair Value, Total                                                                        | 809           |          |
| Gross Unrealized Losses, Total                                                                     | 20            |          |
| Estimated Fair Value, Less than 12 Months                                                          |          | 72            |
| Gross Unrealized Losses, Less than 12 Months                                                       |          | 1             |
| Estimated Fair Value, 12 Months or Longer                                                          |          | 85            |
| Gross Unrealized Losses, 12 Months or Longer                                                       |          | 1             |
| Estimated Fair Value, Total                                                                        |          | 157           |
| Gross Unrealized Losses, Total                                                                     |          | 2             |
| Residential mortgage-backed                                                                        |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 Months                                                          | 3,409         |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 89            |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 501           |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 26            |          |
| Estimated Fair Value, Total                                                                        | 3,910         |          |
| Gross Unrealized Losses, Total                                                                     | 115           |          |
| Estimated Fair Value, Less than 12 Months                                                          |          | 1,228         |
| Gross Unrealized Losses, Less than 12 Months                                                       |          | 5             |
| Estimated Fair Value, 12 Months or Longer                                                          |          | 947           |
| Gross Unrealized Losses, 12 Months or Longer                                                       |          | 27            |
| Estimated Fair Value, Total                                                                        |          | 2,175         |
| Gross Unrealized Losses, Total                                                                     |          | 32            |
| Commercial mortgage-backed                                                                         |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 Months                                                          | 968           |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 15            |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 322           |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 21            |          |
| Estimated Fair Value, Total                                                                        | 1,290         |          |
| Gross Unrealized Losses, Total                                                                     | 36            |          |
| Estimated Fair Value, Less than 12 Months                                                          |          | 403           |
| Gross Unrealized Losses, Less than 12 Months                                                       |          | 4             |
| Estimated Fair Value, 12 Months or Longer                                                          |          | 212           |
| Gross Unrealized Losses, 12 Months or Longer                                                       |          | 10            |
| Estimated Fair Value, Total                                                                        |          | 615           |
| Gross Unrealized Losses, Total                                                                     |          | 14            |
| Other asset-backed                                                                                 |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 Months                                                          | 953           |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 9             |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 45            |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 0             |          |
| Estimated Fair Value, Total                                                                        | 998           |          |
| Gross Unrealized Losses, Total                                                                     | 9             |          |
| Estimated Fair Value, Less than 12 Months                                                          |          | 248           |
| Gross Unrealized Losses, Less than 12 Months                                                       |          | 3             |
| Estimated Fair Value, 12 Months or Longer                                                          |          | 18            |
| Gross Unrealized Losses, 12 Months or Longer                                                       |          | 2             |
| Estimated Fair Value, Total                                                                        |          | 266           |
| Gross Unrealized Losses, Total                                                                     |          | 5             |
| Total asset-backed                                                                                 |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 Months                                                          | 5,330         |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 113           |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 868           |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 47            |          |
| Estimated Fair Value, Total                                                                        | 6,198         |          |
| Gross Unrealized Losses, Total                                                                     | 160           |          |
| Estimated Fair Value, Less than 12 Months                                                          |          | 1,879         |
| Gross Unrealized Losses, Less than 12 Months                                                       |          | 12            |
| Estimated Fair Value, 12 Months or Longer                                                          |          | 1,177         |
| Gross Unrealized Losses, 12 Months or Longer                                                       |          | 39            |
| Estimated Fair Value, Total                                                                        |          | 3,056         |
| Gross Unrealized Losses, Total                                                                     |          | 51            |
| U.S. Treasury and obligations of government-sponsored enterprises                                  |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 Months                                                          | 35            |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 0             |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 42            |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 2             |          |
| Estimated Fair Value, Total                                                                        | 77            |          |
| Gross Unrealized Losses, Total                                                                     | 2             |          |
| Estimated Fair Value, Less than 12 Months                                                          |          | 49            |
| Gross Unrealized Losses, Less than 12 Months                                                       |          | 2             |
| Estimated Fair Value, 12 Months or Longer                                                          |          | 21            |
| Gross Unrealized Losses, 12 Months or Longer                                                       |          | 2             |
| Estimated Fair Value, Total                                                                        |          | 70            |
| Gross Unrealized Losses, Total                                                                     |          | 4             |
| Foreign government                                                                                 |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 Months                                                          | 188           |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 4             |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 64            |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 2             |          |
| Estimated Fair Value, Total                                                                        | 252           |          |
| Gross Unrealized Losses, Total                                                                     | 6             |          |
| Estimated Fair Value, Less than 12 Months                                                          |          | 166           |
| Gross Unrealized Losses, Less than 12 Months                                                       |          | 2             |
| Estimated Fair Value, 12 Months or Longer                                                          |          | 4             |
| Gross Unrealized Losses, 12 Months or Longer                                                       |          | 0             |
| Estimated Fair Value, Total                                                                        |          | 170           |
| Gross Unrealized Losses, Total                                                                     |          | 2             |
| Fixed maturity securities                                                                          |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 Months                                                          | 13,906        |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 333           |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 1,318         |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 70            |          |
| Estimated Fair Value, Total                                                                        | 15,224        |          |
| Gross Unrealized Losses, Total                                                                     | $ 403         |          |
| Estimated Fair Value, Less than 12 Months                                                          |          | 3,520         |
| Gross Unrealized Losses, Less than 12 Months                                                       |          | 38            |
| Estimated Fair Value, 12 Months or Longer                                                          |          | 1,455         |
| Gross Unrealized Losses, 12 Months or Longer                                                       |          | 49            |
| Estimated Fair Value, Total                                                                        |          | 4,975         |
| Gross Unrealized Losses, Total                                                                     |          | 87            |
| Common Stock                                                                                       |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 Months                                                          |          | 7             |
| Gross Unrealized Losses, Less than 12 Months                                                       |          | 1             |
| Estimated Fair Value, 12 Months or Longer                                                          |          | 0             |
| Gross Unrealized Losses, 12 Months or Longer                                                       |          | 0             |
| Estimated Fair Value, Total                                                                        |          | 7             |
| Gross Unrealized Losses, Total                                                                     |          | 1             |
| Preferred stock                                                                                    |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 Months                                                          |          | 93            |
| Gross Unrealized Losses, Less than 12 Months                                                       |          | 1             |
| Estimated Fair Value, 12 Months or Longer                                                          |          | 0             |
| Gross Unrealized Losses, 12 Months or Longer                                                       |          | 0             |
| Estimated Fair Value, Total                                                                        |          | 93            |
| Gross Unrealized Losses, Total                                                                     |          | 1             |
| Equity securities                                                                                  |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                               |          |          |
| Estimated Fair Value, Less than 12 Months                                                          |          | 100           |
| Gross Unrealized Losses, Less than 12 Months                                                       |          | 2             |
| Estimated Fair Value, 12 Months or Longer                                                          |          | 0             |
| Gross Unrealized Losses, 12 Months or Longer                                                       |          | 0             |
| Estimated Fair Value, Total                                                                        |          | 100           |
| Gross Unrealized Losses, Total                                                                     |          | $ 2           |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.
+ References
No definition available.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.
+ References
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| - Definition
This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.
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No definition available.
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| - Definition
Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.
+ References
No definition available.
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| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.
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No definition available.
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| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer.
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No definition available.
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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 more than 12 months.
+ References
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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.
+ References
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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.
+ References
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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.
+ References
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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 unrealized loss position.
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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.
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No definition available.
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