Intangible Assets and Goodwill - Schedule of Indefinite-Lived and Finite-Lived Intangible Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Finite-Lived Intangible Assets [Line Items]
Gross	$ 725	$ 1,135
Accumulated Amortization	(477)	(855)
Net	248	280
Indefinite-lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets	26,641	25,824
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Gross	27,366	26,959
Net	26,889	26,104
Customer relationships and reacquired rights
Indefinite-lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets	22,557	21,740
Visa trade name
Indefinite-lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets	4,084	4,084
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross	535	829
Accumulated Amortization	(298)	(572)
Net	237	257
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross	190	195
Accumulated Amortization	(179)	(172)
Net	11	23
Other
Finite-Lived Intangible Assets [Line Items]
Gross	0	111
Accumulated Amortization	0	(111)
Net	$ 0	$ 0