Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unamortized discount and issuance costs	$ 11	$ 13
Level 2 | Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	752	809
Level 2 | Fair value | Senior Notes | 2024 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	376	401
Level 2 | Fair value | Senior Notes | 2025 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	376	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)