Intangible Assets and Goodwill - Fair Value of Intangible Assets and Related Accumulated Amortization Expense (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Finite-Lived Intangible Assets [Line Items]
Gross	$ 1,020	$ 1,012
Accumulated Amortization	(613)	(533)
Finite-Lived Intangible Assets, Net	407	479
Indefinite-lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets	27,401	26,301
Gross	28,421	27,313
Net	27,808	26,780
Customer relationships and reacquired rights
Indefinite-lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets	23,317	22,217
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	709	701
Accumulated Amortization	(376)	(314)
Finite-Lived Intangible Assets, Net	333	387
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross	199	199
Accumulated Amortization	(134)	(120)
Finite-Lived Intangible Assets, Net	65	79
Reseller relationships
Finite-Lived Intangible Assets [Line Items]
Gross	95	95
Accumulated Amortization	(89)	(86)
Finite-Lived Intangible Assets, Net	6	9
Other
Finite-Lived Intangible Assets [Line Items]
Gross	17	17
Accumulated Amortization	(14)	(13)
Finite-Lived Intangible Assets, Net	$ 3	$ 4