| **Benefit Plans (Estimated Future Minimum Benefit Payments to Participants) (Details) $ in Millions** | Dec. 31, 2017 USD ($) |
|-------------------------------------------------------------------------------------------------------|-----------------------|
| Pension Plans, Defined Benefit                                                                        |                  |
| **Defined Benefit Plan Disclosure [Line Items]**                                                      |                  |
| 2018                                                                                                  | $ 183                 |
| 2019                                                                                                  | 175                   |
| 2020                                                                                                  | 176                   |
| 2021                                                                                                  | 175                   |
| 2022                                                                                                  | 177                   |
| 2023-2027                                                                                             | 871                   |
| Other Postretirement Benefits Plan                                                                    |                  |
| **Defined Benefit Plan Disclosure [Line Items]**                                                      |                  |
| 2018                                                                                                  | 2                     |
| 2019                                                                                                  | 2                     |
| 2020                                                                                                  | 2                     |
| 2021                                                                                                  | 1                     |
| 2022                                                                                                  | 1                     |
| 2023-2027                                                                                             | $ 2                   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of benefits for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following latest fiscal year.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of benefits for defined benefit plan expected to be paid in next fiscal year following latest fiscal year.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of benefits for defined benefit plan expected to be paid in fifth fiscal year following latest fiscal year.
+ References
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| - Definition
Amount of benefits for defined benefit plan expected to be paid in fourth fiscal year following latest fiscal year.
+ References
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| - Definition
Amount of benefits for defined benefit plan expected to be paid in third fiscal year following latest fiscal year.
+ References
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| - Definition
Amount of benefits for defined benefit plan expected to be paid in second fiscal year following latest fiscal year.
+ References
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| X                                                                                                                                                               |
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| X                                                                                                                                                                                  |
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