| **Investments (Contractual maturity) (Details) - USD ($) $ in Millions** | Sep. 30, 2018 | Dec. 31, 2017 |
|--------------------------------------------------------------------------|---------------|---------------|
| **Investments [Abstract]**                                               |          |          |
| Due in one year or less, cost or amortized cost                          | $ 1,457       | $ 1,135       |
| Due after one year through five years, cost or amortized cost            | 8,017         | 8,165         |
| Due after five years through ten years, cost or amortized cost           | 16,453        | 16,060        |
| Due after ten years, cost or amortized cost                              | 12,124        | 12,852        |
| Cost or Amortized Cost                                                   | 38,051        | 38,212        |
| Due in one year or less, estimated fair value                            | 1,473         | 1,157         |
| Due after one year through five years, estimated fair value              | 8,197         | 8,501         |
| Due after five years through ten years, estimated fair value             | 16,475        | 16,718        |
| Due after ten years, estimated fair value                                | 13,482        | 15,108        |
| Total Estimated Fair Value                                               | $ 39,627      | $ 41,484      |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.
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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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