| **Investments (Summary of fixed maturity and equity securities) (Details) - USD ($) $ in Millions** | Dec. 31, 2017 | Dec. 31, 2016 |
|-----------------------------------------------------------------------------------------------------|---------------|---------------|
| Cost or Amortized Cost                                                                              | $ 38,874      | $ 38,467      |
| Gross Unrealized Gains                                                                              | 3,397         | 2,744         |
| Gross Unrealized Losses                                                                             | 89            | 196           |
| Estimated Fair Value                                                                                | 42,182        | 41,015        |
| Corporate and other bonds                                                                           |          |          |
| Cost or Amortized Cost                                                                              | 17,210        | 17,711        |
| Gross Unrealized Gains                                                                              | 1,625         | 1,323         |
| Gross Unrealized Losses                                                                             | 28            | 76            |
| Estimated Fair Value                                                                                | 18,807        | 18,958        |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | (1)           |
| States, municipalities and political subdivisions                                                   |          |          |
| Cost or Amortized Cost                                                                              | 12,478        | 12,060        |
| Gross Unrealized Gains                                                                              | 1,551         | 1,213         |
| Gross Unrealized Losses                                                                             | 2             | 33            |
| Estimated Fair Value                                                                                | 14,027        | 13,240        |
| Unrealized OTTI Losses (Gains)                                                                      | (11)          | (16)          |
| Residential mortgage-backed                                                                         |          |          |
| Cost or Amortized Cost                                                                              | 5,043         | 5,004         |
| Gross Unrealized Gains                                                                              | 109           | 120           |
| Gross Unrealized Losses                                                                             | 32            | 51            |
| Estimated Fair Value                                                                                | 5,120         | 5,073         |
| Unrealized OTTI Losses (Gains)                                                                      | (27)          | (28)          |
| Commercial mortgage-backed                                                                          |          |          |
| Cost or Amortized Cost                                                                              | 1,840         | 2,016         |
| Gross Unrealized Gains                                                                              | 46            | 48            |
| Gross Unrealized Losses                                                                             | 14            | 24            |
| Estimated Fair Value                                                                                | 1,872         | 2,040         |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Other asset-backed                                                                                  |          |          |
| Cost or Amortized Cost                                                                              | 1,083         | 1,022         |
| Gross Unrealized Gains                                                                              | 16            | 8             |
| Gross Unrealized Losses                                                                             | 5             | 5             |
| Estimated Fair Value                                                                                | 1,094         | 1,025         |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Total asset-backed                                                                                  |          |          |
| Cost or Amortized Cost                                                                              | 7,966         | 8,042         |
| Gross Unrealized Gains                                                                              | 171           | 176           |
| Gross Unrealized Losses                                                                             | 51            | 80            |
| Estimated Fair Value                                                                                | 8,086         | 8,138         |
| Unrealized OTTI Losses (Gains)                                                                      | (27)          | (28)          |
| U.S. Treasury and obligations of government-sponsored enterprises                                   |          |          |
| Cost or Amortized Cost                                                                              | 111           | 83            |
| Gross Unrealized Gains                                                                              | 2             | 10            |
| Gross Unrealized Losses                                                                             | 4             | 0             |
| Estimated Fair Value                                                                                | 109           | 93            |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Foreign government                                                                                  |          |          |
| Cost or Amortized Cost                                                                              | 437           | 435           |
| Gross Unrealized Gains                                                                              | 9             | 13            |
| Gross Unrealized Losses                                                                             | 2             | 3             |
| Estimated Fair Value                                                                                | 444           | 445           |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Redeemable preferred stock                                                                          |          |          |
| Cost or Amortized Cost                                                                              | 10            | 18            |
| Gross Unrealized Gains                                                                              | 1             | 1             |
| Gross Unrealized Losses                                                                             | 0             | 0             |
| Estimated Fair Value                                                                                | 11            | 19            |
| Unrealized OTTI Losses (Gains)                                                                      | 0             | 0             |
| Fixed maturity securities                                                                           |          |          |
| Cost or Amortized Cost                                                                              | 38,212        | 38,349        |
| Gross Unrealized Gains                                                                              | 3,359         | 2,736         |
| Gross Unrealized Losses                                                                             | 87            | 192           |
| Estimated Fair Value                                                                                | 41,484        | 40,893        |
| Unrealized OTTI Losses (Gains)                                                                      | (38)          | (45)          |
| Total fixed maturity securities trading                                                             |          |          |
| Cost or amortized cost, trading securities                                                          | 3             | 12            |
| Estimated fair value, trading securities                                                            | 3             | 12            |
| Common Stock                                                                                        |          |          |
| Cost or Amortized Cost                                                                              | 21            | 13            |
| Gross Unrealized Gains                                                                              | 7             | 6             |
| Gross Unrealized Losses                                                                             | 1             | 0             |
| Estimated Fair Value                                                                                | 27            | 19            |
| Preferred stock                                                                                     |          |          |
| Cost or Amortized Cost                                                                              | 638           | 93            |
| Gross Unrealized Gains                                                                              | 31            | 2             |
| Gross Unrealized Losses                                                                             | 1             | 4             |
| Estimated Fair Value                                                                                | 668           | 91            |
| Equity securities                                                                                   |          |          |
| Cost or Amortized Cost                                                                              | 659           | 106           |
| Gross Unrealized Gains                                                                              | 38            | 8             |
| Gross Unrealized Losses                                                                             | 2             | 4             |
| Estimated Fair Value                                                                                | $ 695         | $ 110         |

| X                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The accumulated unrealized gain or loss position on Other Than Temporary Impairment (OTTI) Securities
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cost of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.
+ References
No definition available.
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