| **Investments (Contractual maturity) (Details) - USD ($) $ in Millions** | Mar. 31, 2020 | Dec. 31, 2019 |
|--------------------------------------------------------------------------|---------------|---------------|
| **Investments [Abstract]**                                               |          |          |
| Due in one year or less, cost or amortized cost                          | $ 1,331       | $ 1,334       |
| Due after one year through five years, cost or amortized cost            | 11,554        | 9,746         |
| Due after five years through ten years, cost or amortized cost           | 13,078        | 14,892        |
| Due after ten years, cost or amortized cost                              | 12,056        | 12,134        |
| Cost or Amortized Cost                                                   | 38,019        | 38,106        |
| Due in one year or less, estimated fair value                            | 1,325         | 1,356         |
| Due after one year through five years, estimated fair value              | 11,812        | 10,186        |
| Due after five years through ten years, estimated fair value             | 13,069        | 15,931        |
| Due after ten years, estimated fair value                                | 13,877        | 14,714        |
| Total Estimated Fair Value                                               | $ 40,083      | $ 42,187      |

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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
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| - Definition
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| - Definition
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| - Definition
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| - Definition
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| - Definition
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| - Definition
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| - Definition
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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).
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