Legal Matters - Additional Information (Detail) $ in Millions				1 Months Ended				3 Months Ended	8 Months Ended	12 Months Ended		48 Months Ended	60 Months Ended
	Sep. 17, 2018 USD ($)	Jan. 27, 2014 USD ($)	Dec. 10, 2012 USD ($)	Apr. 30, 2018 case_filed	Nov. 30, 2016 litigation_case	Oct. 31, 2011 financial_institution	Dec. 31, 2010 financial_institution entity	Sep. 30, 2014 USD ($)	Aug. 10, 2013 USD ($)	Sep. 30, 2018 USD ($) province case_filed merchant	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) merchant	Sep. 30, 2018 USD ($) case_filed merchant	Sep. 16, 2018 USD ($)	Sep. 30, 2016 USD ($)	Oct. 11, 2011 atm_operator
Loss Contingencies [Line Items]
Deposits into litigation escrow account										$ 600	$ 0
Period of accrual	5 years
Loss contingency accrual										$ 1,434	982	$ 982	$ 1,434		$ 981
Percentage of class members opt out	15.00%
Possible return to defendants	$ 467
Number of separate cases | case_filed				2
Number of provinces with cases | province										5
Class Plaintiffs
Loss Contingencies [Line Items]
Payments for legal settlements			$ 4,000
Settlement, Further Distribution of Default Interchange, Rate									$ 500
Loss contingency, further distributions, default period									8 months
Loss Contingency, Number of Cases Appointed Counsel | litigation_case					2
Termination Of Amended Settlement [Member]
Loss Contingencies [Line Items]
Percentage of class members opt out	25.00%
Interchange Opt Out Litigation
Loss Contingencies [Line Items]
Number of opt-out cases filed | case_filed										50			50
U.K. Merchant Litigation
Loss Contingencies [Line Items]
Number of plaintiffs | merchant													400
Damages sought, value													$ 2,000
Canadian Competition Proceedings
Loss Contingencies [Line Items]
Number of Financial Institutions | financial_institution							10
National ATM Council Class Action
Loss Contingencies [Line Items]
Number of non-bank ATM operators | atm_operator																13
Consumer Class Actions
Loss Contingencies [Line Items]
Number of Financial Institutions | financial_institution						3
Number of claims pending | financial_institution						2
Settled Litigation | U.K. Merchant Litigation
Loss Contingencies [Line Items]
Number of plaintiffs | merchant												75
Settled Litigation | Canadian Competition Proceedings
Loss Contingencies [Line Items]
Number of Financial Institutions | entity							4
U.S. Covered Litigation | Interchange Multidistrict Litigation
Loss Contingencies [Line Items]
Provision for legal matters		$ 1,100
Pending Litigation | U.K. Merchant Litigation
Loss Contingencies [Line Items]
Number of plaintiffs | merchant												300
Threatened Litigation | Europe Interchange Rate Litigation
Loss Contingencies [Line Items]
Number of plaintiffs | merchant										30
Visa, MasterCard, and Certain U.S. Financial Institutions
Loss Contingencies [Line Items]
Loss contingency accrual														$ 5,300
Possible return to defendants	$ 700
U.S. Covered Litigation
Loss Contingencies [Line Items]
Deposits into litigation escrow account								$ 450
Loss contingency accrual										$ 1,428	978	$ 978	$ 1,428		$ 978
U.S. Covered Litigation | Interchange Multidistrict Litigation
Loss Contingencies [Line Items]
Provision for legal matters										$ 600	$ 0
U.S. Covered Litigation | Visa, MasterCard, and Certain U.S. Financial Institutions | Interchange Multidistrict Litigation
Loss Contingencies [Line Items]
Provision for legal matters	$ 900