Receivables, Loans, Notes Receivable, and Others (Details) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2021	Sep. 30, 2020
Accounts and Financing Receivable, after Allowance for Credit Loss, Current and Noncurrent [Abstract]
Accounts receivable	$ 5,315	$ 5,467
Less: Allowance for expected credit losses	(148)	(173)
Accounts receivable - net	5,167	$ 5,294
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	173
Provision for expected credit losses	3
Write-offs charged against the allowance for expected credit losses	(32)
Other (including impact of adoption of ASU 2016-13)	4
Accounts Receivable, Allowance for Credit Loss, Ending Balance	$ 148