Significant Accounting Policies - Schedule of Change Retrospectively to Prior Periods Consolidated Statements of Cash Flows (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Significant Accounting Policies [Line Items]
Net income (loss)	$ 102	$ 99	$ (163)	$ (112)	$ 79	$ 55	$ 80	$ 59	$ (74)	$ 273	$ 162
Deferred tax provision (benefit)									91	(31)	(36)
Inventories									$ (105)	(61)	54
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)										273	162
Deferred tax provision (benefit)										(31)	(36)
Inventories										(61)	54
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)										266	164
Deferred tax provision (benefit)										(35)	(35)
Inventories										(50)	51
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)										7	(2)
Deferred tax provision (benefit)										4	(1)
Inventories										$ (11)	$ 3