Restructuring - Restructuring Activities and Related Reserves (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Restructuring Cost And Reserve [Line Items]
Reserve balance	$ 7	$ 5	$ 3
Charges (gain)	19	16	(30)
Costs charged against assets (liabilities)	(1)	(2)	(2)
Cash (paid) received	(16)	(12)	34
Reserve balance	9	7	5
Severance and Employee Benefits [Member]
Restructuring Cost And Reserve [Line Items]
Reserve balance	3	1	1
Charges (gain)	14	11	2
Cash (paid) received	(12)	(9)	(2)
Reserve balance	5	3	1
Environmental Remediation and Decommissioning Activities [Member]
Restructuring Cost And Reserve [Line Items]
Reserve balance	4	4	2
Charges (gain)			3
Cash (paid) received			(1)
Reserve balance	4	4	4
Non-Cash Asset Impairment and Accelerated Depreciation [Member]
Restructuring Cost And Reserve [Line Items]
Charges (gain)	1	2	1
Costs charged against assets (liabilities)	(1)	(2)	(1)
Asset Sales [Member]
Restructuring Cost And Reserve [Line Items]
Charges (gain)			(38)
Costs charged against assets (liabilities)			(1)
Cash (paid) received			39
Other [Member]
Restructuring Cost And Reserve [Line Items]
Charges (gain)	4	3	2
Cash (paid) received	$ (4)	$ (3)	$ (2)