U.S. and Europe Retrospective Responsibility Plans - Changes in the U.S. Litigation Escrow Account (Detail) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Escrow Account [Roll Forward]
Balance at beginning of period	$ 901	$ 1,205
Return of takedown payment to the litigation escrow account	0	467
Balance at end of period	894	1,264
Interest Income | Opt-out Merchants
Escrow Account [Roll Forward]
Payments to opt-out merchants and interest earned on escrow funds	$ (7)	$ (408)