Financial Information by Segment - Sales of Equity Method Affiliate and Discounting Charges for Certain Notes Receivable, by-product Revenue and Indirect Tax Settlement Credits (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Revenues from external customers	$ 917	$ 518	$ 2,505	$ 1,955
Unallocated and Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	66	38	162	131
Unallocated and Other [Member] | Shipping and Handling Costs [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	41	22	112	83
Unallocated and Other [Member] | By-product Sales [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	19	12	54	46
Unallocated and Other [Member] | Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	$ 6	$ 4	$ (4)	$ 2