Leases - Supplemental Balance Sheet Information Related to Company's Leases (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Lease ROU assets:
Operating	$ 90	$ 98
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	http://fasb.org/us-gaap/2021-01-31#OtherAssetsNoncurrent	http://fasb.org/us-gaap/2021-01-31#OtherAssetsNoncurrent
Finance	$ 44	$ 44
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	http://fasb.org/us-gaap/2021-01-31#PropertyPlantAndEquipmentNet	http://fasb.org/us-gaap/2021-01-31#PropertyPlantAndEquipmentNet
Total lease ROU assets	$ 134	$ 142
Lease liabilities:
Operating	$ 14	$ 15
Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	http://fasb.org/us-gaap/2021-01-31#AccountsPayableAndAccruedLiabilitiesCurrent	http://fasb.org/us-gaap/2021-01-31#AccountsPayableAndAccruedLiabilitiesCurrent
Finance	$ 4	$ 3
Finance Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	http://fasb.org/us-gaap/2021-01-31#LongTermDebtCurrent	http://fasb.org/us-gaap/2021-01-31#LongTermDebtCurrent
Operating	$ 84	$ 89
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]	http://fasb.org/us-gaap/2021-01-31#OtherLiabilitiesNoncurrent	http://fasb.org/us-gaap/2021-01-31#OtherLiabilitiesNoncurrent
Finance	$ 29	$ 28
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]	http://fasb.org/us-gaap/2021-01-31#LongTermDebtNoncurrent	http://fasb.org/us-gaap/2021-01-31#LongTermDebtNoncurrent
Total lease liabilities	$ 131	$ 135