| **Fair Value Measurements and Disclosures - Fair Value of Debt (Details) - USD ($) $ in Millions** | Sep. 30, 2018 | Dec. 31, 2017 |
|----------------------------------------------------------------------------------------------------|---------------|---------------|
| Carrying Amount                                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**   |          |          |
| Long-term Debt, Net                                                                                | $ 6,442       | $ 6,586       |
| Fair Value(1)                                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**   |          |          |
| Long-term Debt, Net                                                                                | $ 6,498       | $ 7,142       |

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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 fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.
+ References
No definition available.
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