Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unamortized discount and issuance costs	$ 13	$ 9
Level 2 | Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	944	797
Level 2 | Fair value | Senior Notes | 2024 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	0	390
Level 2 | Fair value | Senior Notes | 2025 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	384	407
Level 2 | Fair value | Senior Notes | Senior Unsecured Notes Due 2030 [Domain]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	560	0
Level 2 | Carrying value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	988	766
Unamortized discount and issuance costs	(12)	(9)
Level 2 | Carrying value | Senior Notes | 2024 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	0	371
Unamortized discount and issuance costs	0	(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)
Level 2 | Carrying value | Senior Notes | Senior Unsecured Notes Due 2030 [Domain]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	592	0
Unamortized discount and issuance costs	$ (8)	$ 0