Restructuring - Restructuring Activities and Related Reserves (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Restructuring Cost And Reserve [Line Items]
Reserve balance	$ 5	$ 3	$ 5
Charges (gain)	16	(30)	3
Costs charged against assets (liabilities)	(2)	(2)	1
Cash (paid) received	(12)	34	(6)
Reserve balance	7	5	3
Severance and Employee Benefits [Member]
Restructuring Cost And Reserve [Line Items]
Reserve balance	1	1	3
Charges (gain)	11	2	1
Cash (paid) received	(9)	(2)	(3)
Reserve balance	3	1	1
Environmental Remediation and Decommissioning Activities [Member]
Restructuring Cost And Reserve [Line Items]
Reserve balance	4	2	2
Charges (gain)		3	1
Cash (paid) received		(1)	(1)
Reserve balance	4	4	2
Non-Cash Asset Impairment and Accelerated Depreciation [Member]
Restructuring Cost And Reserve [Line Items]
Charges (gain)	2	1
Costs charged against assets (liabilities)	(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)	3	2	1
Costs charged against assets (liabilities)			1
Cash (paid) received	$ (3)	$ (2)	$ (2)