Significant Accounting Policies - Schedule of Change Retrospectively to Prior Periods Consolidated Statements of Cash Flows (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Significant Accounting Policies [Line Items]
Net income (loss)	$ (163)	$ 80	$ (275)	$ 139
Deferred tax provision (benefit)			157	(26)
Inventories			$ (61)	(65)
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]
Net income (loss)		80		139
Deferred tax provision (benefit)				(26)
Inventories				(65)
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]
Net income (loss)		$ 80		138
Deferred tax provision (benefit)				(27)
Inventories				(63)
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]
Net income (loss)				1
Deferred tax provision (benefit)				1
Inventories				$ (2)