Accrued Litigation for Both Covered and Non-Covered Litigation (Detail) $ in Millions	6 Months Ended		69 Months Ended	70 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2019 USD ($) merchant	Apr. 26, 2019 merchant	Dec. 04, 2018
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	$ 1,434	$ 982
Provision for legal matters	159	1
Balance at end of period	914	830	$ 914
Interchange Rate, Card-Not-Present, Credit					0.015
Interchange Rate, Card-Not-Present, Debit					0.0115
Interchange Rate, Card Present, Credit					0.003
Interchange Rate, Card Present, Debit					0.002
U.K. Merchant Litigation
Loss Contingency Accrual [Roll Forward]
Number of plaintiffs | merchant			450
Unsettled
Loss Contingency Accrual [Roll Forward]
Provision for legal matters	35	0
Settled Litigation
Loss Contingency Accrual [Roll Forward]
Loss Contingency Accrual, Payments	(714)	(153)
Threatened Litigation | U.K. Merchant Litigation
Loss Contingency Accrual [Roll Forward]
Number of plaintiffs | merchant			30
U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	1,428	978
Balance at end of period	828	828	$ 828
U.S. Covered Litigation | Settled Litigation
Loss Contingency Accrual [Roll Forward]
Loss Contingency Accrual, Payments	(600)	(150)
VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	0	1
Provision for legal matters	159	1
Balance at end of period	61	0	$ 61
VE Territory Covered Litigation | Settled Litigation
Loss Contingency Accrual [Roll Forward]
Loss Contingency Accrual, Payments	$ (98)	$ (2)
Subsequent Event | U.K. Merchant Litigation
Loss Contingency Accrual [Roll Forward]
Number of claims settled | merchant				100
Merchants with outstanding claims | merchant				350