Financial Information by Segment & Geographic Area - Sales of Equity Method Affiliate and Discounting Charges for Certain Notes Receivable (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues from external customers	$ 659	$ 518	$ 710	$ 727	$ 827	$ 845	$ 844	$ 821	$ 2,614	$ 3,337	$ 3,242
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									172	193	116
Unallocated and Other [Member] | Royalties, the Impact of Unearned Revenue, the Removal of 100% of the Sales of an Equity Method Affiliate and Discounting Charges for Certain Notes Receivable [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									(3)	(13)	11
Unallocated and Other [Member] | Shipping and Handling Costs [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									113	130	$ 105
Unallocated and Other [Member] | By-product Sales [Member]
Segment Reporting Information [Line Items]
Revenues from external customers									$ 62	$ 76