U.S. and Europe Retrospective Responsibility Plans - Changes in the U.S. Litigation Escrow Account (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Escrow Account [Roll Forward]
Beginning balance	$ 1,205	$ 1,491
Return of takedown payment to the litigation escrow account	467	0
Deposits into the litigation escrow account	0	300	$ 600
Payments to class plaintiffs settlement funds	0	(600)
Ending balance	901	1,205	$ 1,491
Interest Income | Opt-out Merchants
Escrow Account [Roll Forward]
Payments to opt-out merchants and interest earned on escrow funds	$ (771)	$ 14