Significant Accounting Policies - Schedule of Change Retrospectively to Prior Periods Consolidated Statements of Operations (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Significant Accounting Policies [Line Items]
Cost of sales	$ 666	$ 630	$ 1,321	$ 1,174
Gross profit	178	188	344	364
Selling and administrative expenses	70	79	143	149
Research and technical expenses	15	16	31	31
Income (loss) from operations	73	(161)	150	(70)
Other income (expense)	$ (12)	1	$ (6)	12
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		630		1,174
Gross profit		188		364
Selling and administrative expenses		79		149
Research and technical expenses		16		31
Income (loss) from operations		(161)		(70)
Other income (expense)		1		12
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		628		1,170
Gross profit		190		368
Selling and administrative expenses		78		147
Research and technical expenses		16		31
Income (loss) from operations		(158)		(64)
Other income (expense)		(2)		6
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		2		4
Gross profit		(2)		(4)
Selling and administrative expenses		1		2
Income (loss) from operations		(3)		(6)
Other income (expense)		$ 3		$ 6