Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unamortized discounts and issuance costs	$ 14	$ 19
Level 2 | Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	1,153	1,440
Level 2 | Fair value | Senior Notes | 2025 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	0	827
Level 2 | Fair value | Senior Notes | Senior Unsecured Notes Due 2030
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	622	613
Level 2 | Fair value | Senior Notes | Senior Unsecured Notes Due 2031
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	531	0
Level 2 | Carrying value (a)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	1,122	1,382
Unamortized discounts and issuance costs	(13)	(18)
Level 2 | Carrying value (a) | Senior Notes | 2025 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	0	790
Unamortized discounts and issuance costs	0	(10)
Level 2 | Carrying value (a) | Senior Notes | Senior Unsecured Notes Due 2030
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	593	592
Unamortized discounts and issuance costs	(7)	(8)
Level 2 | Carrying value (a) | Senior Notes | Senior Unsecured Notes Due 2031
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	529	0
Unamortized discounts and issuance costs	$ (6)	$ 0