U.S. and Europe Retrospective Responsibility Plans - Schedule of Changes in the U.S. Litigation Escrow Account (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Escrow Account [Roll Forward]
Balance as of beginning of period	$ 1,764	$ 1,449
Deposits into the U.S. litigation escrow account	1,500	1,000	$ 850
Balance as of end of period	3,089	1,764	$ 1,449
Interest Income | Opt-out Merchants
Escrow Account [Roll Forward]
Payments to opt-out merchants, net of interest earned on escrow funds	$ (175)	$ (685)