| **Postretirement Benefits - Components of Postretirement Benefits Included in Operations (Details) - USD ($) $ in Millions** | 3 Months Ended | 6 Months Ended |
|------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
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| Pension Benefits                                                                                                             |           |           |   |   |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                             |           |           |   |   |
| Net postretirement benefits expense                                                                                          | $ 22           | $ 17           | $ 42 | $ 33 |
| Health and Life Insurance Benefits                                                                                           |           |           |   |   |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                             |           |           |   |   |
| Net postretirement benefits expense                                                                                          | $ 17           | $ 19           | $ 35 | $ 40 |

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| - 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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| - Definition
The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920
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| X                                                                                                                                                                                              |
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| X                                                                                                                                                                                                                 |
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