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Fair Value Measurements - Fair Value of Debt (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Sep. 30, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Unamortized discount and issuance costs $ 12 $ 13
Quoted prices in active markets for identical assets, Level 1 | Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value 803 809
Quoted prices in active markets for identical assets, Level 1 | Fair Value | Senior Notes | 2024 Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value 399 401
Quoted prices in active markets for identical assets, Level 1 | Fair Value | Senior Notes | 2025 Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value 404 408
Quoted prices in active markets for identical assets, Level 1 | Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value 765 764
Unamortized discount and issuance costs 10 11
Quoted prices in active markets for identical assets, Level 1 | Carrying Value | Senior Notes | 2024 Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value 370 370
Unamortized discount and issuance costs 5 5
Quoted prices in active markets for identical assets, Level 1 | Carrying Value | Senior Notes | 2025 Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value 395 394
Unamortized discount and issuance costs $ 5 $ 6