U.S. and Europe Retrospective Responsibility Plans - Schedule of Changes in the U.S. Litigation Escrow Account (Detail) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Escrow Account [Roll Forward]
Balance as of beginning of period	$ 3,089	$ 1,764
Deposits into the U.S. litigation escrow account	375	0
Balance as of end of period	2,926	1,584
Interest Income | Opt-out Merchants
Escrow Account [Roll Forward]
Payments to opt-out merchants, net of interest earned on escrow funds	$ (538)	$ (180)