Changes in the U.S. Litigation Escrow Account (Detail) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Deposits into litigation escrow account		$ 600	$ 0
Escrow Account [Roll Forward]
Litigation Escrow Account Balance, Beginning of Period		1,031	1,027
Litigation Escrow Account Balance, End of Period		1,491	1,031
Interest Income | Opt-out Merchants
Escrow Account [Roll Forward]
Payments to opt-out merchants and interest earned on escrow funds	[1]	$ (140)	$ 4

[1] 1)These payments are associated with the interchange multidistrict litigation. See Note 17—Legal Matters.