Fair Value Measurements - Fair Value of Debt (Details) - USD ($) $ in Millions	Jun. 30, 2021	Sep. 30, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unamortized discounts and issuance costs	$ 14	$ 19
Fair Value, Inputs, Level 2 [Member] | Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	1,156	1,440
Fair Value, Inputs, Level 2 [Member] | Fair value | Senior Notes | 2025 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	0	827
Fair Value, Inputs, Level 2 [Member] | 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	621	613
Fair Value, Inputs, Level 2 [Member] | Fair value | Senior Notes | Senior Unsecured Notes Due 2031 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	535	0
Fair Value, Inputs, Level 2 [Member] | Carrying value
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)
Fair Value, Inputs, Level 2 [Member] | Carrying value | 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)
Fair Value, Inputs, Level 2 [Member] | 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	593	592
Unamortized discounts and issuance costs	(7)	(8)
Fair Value, Inputs, Level 2 [Member] | Carrying value | Senior Notes | Senior Unsecured Notes Due 2031 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	529	0
Unamortized discounts and issuance costs	$ (6)	$ 0