U.S. and Europe Retrospective Responsibility Plans - Changes in the U.S. Litigation Escrow Account (Detail) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Escrow Account [Roll Forward]
Balance as of beginning of period	$ 1,764	$ 1,449
Deposits into the U.S. litigation escrow account	0	350
Balance as of end of period	1,584	1,616
Interest Income | Opt-out Merchants
Escrow Account [Roll Forward]
Payments to opt-out merchants, net of interest earned on escrow funds	$ (180)	$ (183)