Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unamortized discount and issuance costs	$ 9	$ 9
Level 2 | Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	805	797
Level 2 | Fair value | Senior Notes | 2024 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	389	390
Level 2 | Fair value | Senior Notes | 2025 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	416	407
Level 2 | Carrying value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	767	766
Unamortized discount and issuance costs	(8)	(9)
Level 2 | Carrying value | Senior Notes | 2024 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	371	371
Unamortized discount and issuance costs	(4)	(4)
Level 2 | Carrying value | Senior Notes | 2025 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	396	395
Unamortized discount and issuance costs	$ (4)	$ (5)