Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unamortized discount and issuance costs	$ 12	$ 13
Level 2 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	774	809
Level 2 | Fair Value | Senior Notes | 2024 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	385	401
Level 2 | Fair Value | Senior Notes | 2025 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	389	408
Level 2 | 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
Level 2 | 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
Level 2 | 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