Restructuring - Restructuring Activities and Related Reserves (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Restructuring Cost And Reserve [Line Items]
Reserve balance	$ 3	$ 5	$ 9
Charges (gain)	(30)	3	47
Costs charged against liabilities (assets)	(2)	1	(30)
Cash (paid) received	34	(6)	(21)
Reserve balance	5	3	5
Severance and Employee Benefits [Member]
Restructuring Cost And Reserve [Line Items]
Reserve balance	1	3	5
Charges (gain)	2	1	28
Cash (paid) received	(2)	(3)	(30)
Reserve balance	1	1	3
Environmental Remediation and Decommissioning Activities [Member]
Restructuring Cost And Reserve [Line Items]
Reserve balance	2	2	2
Charges (gain)	3	1
Cash (paid) received	(1)	(1)
Reserve balance	4	2	2
Non-Cash Asset Impairment and Accelerated Depreciation [Member]
Restructuring Cost And Reserve [Line Items]
Charges (gain)	1		23
Costs charged against liabilities (assets)	(1)		(23)
Asset Sales [Member]
Restructuring Cost And Reserve [Line Items]
Charges (gain)	(38)		(9)
Costs charged against liabilities (assets)	(1)		(7)
Cash (paid) received	39		16
Other [Member]
Restructuring Cost And Reserve [Line Items]
Reserve balance			2
Charges (gain)	2	1	5
Costs charged against liabilities (assets)		1
Cash (paid) received	$ (2)	$ (2)	$ (7)