| **Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions** | Jun. 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                                                          | $ 6,963       |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 192           |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 193           |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 11            |          |
| Estimated Fair Value, Total                                                                        | 7,156         |          |
| Gross Unrealized Losses, Total                                                                     | 203           |          |
| 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                                                          | 723           |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 8             |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 3             |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 0             |          |
| Estimated Fair Value, Total                                                                        | 726           |          |
| Gross Unrealized Losses, Total                                                                     | 8             |          |
| 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                                                          | 3,183         |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 68            |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 344           |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 17            |          |
| Estimated Fair Value, Total                                                                        | 3,527         |          |
| Gross Unrealized Losses, Total                                                                     | 85            |          |
| 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                                                          | 913           |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 16            |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 228           |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 17            |          |
| Estimated Fair Value, Total                                                                        | 1,141         |          |
| Gross Unrealized Losses, Total                                                                     | 33            |          |
| 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                                                          | 632           |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 6             |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 27            |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 3             |          |
| Estimated Fair Value, Total                                                                        | 659           |          |
| 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                                                          | 4,728         |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 90            |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 599           |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 37            |          |
| Estimated Fair Value, Total                                                                        | 5,327         |          |
| Gross Unrealized Losses, Total                                                                     | 127           |          |
| 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                                                          | 49            |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 1             |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 19            |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 2             |          |
| Estimated Fair Value, Total                                                                        | 68            |          |
| Gross Unrealized Losses, Total                                                                     | 3             |          |
| 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                                                          | 178           |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 3             |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 40            |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 2             |          |
| Estimated Fair Value, Total                                                                        | 218           |          |
| Gross Unrealized Losses, Total                                                                     | 5             |          |
| 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                                                          | 12,641        |          |
| Gross Unrealized Losses, Less than 12 Months                                                       | 294           |          |
| Estimated Fair Value, 12 Months or Longer                                                          | 854           |          |
| Gross Unrealized Losses, 12 Months or Longer                                                       | 52            |          |
| Estimated Fair Value, Total                                                                        | 13,495        |          |
| Gross Unrealized Losses, Total                                                                     | $ 346         |          |
| 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.
+ 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 unrealized loss position.
+ References
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No definition available.
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