Significant Accounting Policies - Schedule of Change Retrospectively to Prior Periods Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Oct. 01, 2015
Significant Accounting Policies [Line Items]
Inventories	$ 511	$ 433
Deferred income taxes (assets)	134	237
Retained earnings	$ 1,417	1,707
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member]
Significant Accounting Policies [Line Items]
Inventories		433	$ 368
Deferred income taxes (assets)		237	207
Retained earnings		1,707	1,561	$ 1,497
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member] | As Originally Reported [Member]
Significant Accounting Policies [Line Items]
Inventories		396	342
Deferred income taxes (assets)		250	216
Retained earnings		1,683	1,544	1,478
Change in Method of Accounting for Inventories from Last-in, First-out Method to First-in, First-out Method [Member] | Effect of Change [Member]
Significant Accounting Policies [Line Items]
Inventories		37	26
Deferred income taxes (assets)		(13)	(9)
Retained earnings		$ 24	$ 17	$ 19