Accrued Litigation for Both Covered and Non-Covered Litigation (Detail) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended		10 Months Ended
	Mar. 31, 2017 merchant	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($) merchant	Jun. 30, 2017 USD ($)	Jul. 27, 2018 USD ($) merchant
Loss Contingencies [Line Items]
Deposits To Litigation Escrow Account		$ 600	$ 600
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period			982	$ 981	$ 982
Provision for legal matters			601	142
Balance at end of period		1,428	$ 1,428	995
U.K. Merchant Litigation
Loss Contingencies [Line Items]
Number of plaintiffs | merchant			400
Number of merchant claims not settled before trial | merchant	1
Unsettled
Loss Contingency Accrual [Roll Forward]
Provision for legal matters			$ 0	17
Settled Litigation
Loss Contingency Accrual [Roll Forward]
Loss Contingency Accrual, Payments			$ (155)	(145)
Threatened Litigation | U.K. Merchant Litigation
Loss Contingencies [Line Items]
Number of plaintiffs | merchant			30
U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period			$ 978	978	978
Balance at end of period		1,428	1,428	978
U.S. Covered Litigation | Unsettled
Loss Contingency Accrual [Roll Forward]
Provision for legal matters			600	0
U.S. Covered Litigation | Settled Litigation
Loss Contingency Accrual [Roll Forward]
Loss Contingency Accrual, Payments			(150)	0
VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period			1	2	$ 1
Provision for legal matters			1	142
Balance at end of period		$ 0	0	0
VE Territory Covered Litigation | Settled Litigation
Loss Contingency Accrual [Roll Forward]
Loss Contingency Accrual, Payments			$ (2)	$ (144)
Subsequent Event | U.K. Merchant Litigation
Loss Contingencies [Line Items]
Number of claims settled | merchant					75
Merchants with outstanding claims | merchant					300
Subsequent Event | Interchange Multidistrict Litigation
Loss Contingencies [Line Items]
Merchants with settlement agreements (percent)					51.00%