Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unamortized discount and issuance costs	$ 10	$ 11
Level 2 | Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	789	752
Level 2 | Fair value | Senior Notes | 2024 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	388	376
Level 2 | Fair value | Senior Notes | 2025 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	401	376
Level 2 | Carrying value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	766	765
Unamortized discount and issuance costs	9	10
Level 2 | Carrying value | Senior Notes | 2024 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	371	370
Unamortized discount and issuance costs	4	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	395
Unamortized discount and issuance costs	$ 5	$ 5