Significant Accounting Policies - Schedule of Change Retrospectively to Prior Periods Consolidated Statements of Operations (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Significant Accounting Policies [Line Items]
Cost of sales	$ 675	$ 657	$ 1,996	$ 1,831
Gross profit	170	197	514	561
Selling and administrative expenses	65	74	208	223
Research and technical expenses	16	17	47	48
Income (loss) from operations	$ 81	106	231	36
Other income (expense)		1	$ (6)	13
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]
Cost of sales		657		1,831
Gross profit		197		561
Selling and administrative expenses		74		223
Research and technical expenses		17		48
Income (loss) from operations		106		36
Other income (expense)		1		13
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]
Cost of sales		654		1,824
Gross profit		200		568
Selling and administrative expenses		74		221
Research and technical expenses		17		48
Income (loss) from operations		109		45
Other income (expense)		(2)		4
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]
Cost of sales		3		7
Gross profit		(3)		(7)
Selling and administrative expenses				2
Income (loss) from operations		(3)		(9)
Other income (expense)		$ 3		$ 9