Legal Matters - Additional Information (Detail) $ in Millions				1 Months Ended			12 Months Ended			24 Months Ended	76 Months Ended	78 Months Ended
	Sep. 17, 2018 USD ($)	Jun. 30, 2016 litigation_case	Jan. 27, 2014 USD ($)	Apr. 30, 2018 case_filed	Oct. 31, 2011 atm_operator financial_institution	Dec. 31, 2010 financial_institution	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($) province	Sep. 30, 2012 USD ($)	Sep. 30, 2016 entity	Nov. 14, 2019 merchant	Nov. 14, 2019 case_filed merchant	Sep. 30, 2017 USD ($)
Loss Contingencies [Line Items]
Deposits into the litigation escrow account							$ 300	$ 600
Loss contingency accrual							1,203	$ 1,434					$ 982
Class Plaintiffs
Loss Contingencies [Line Items]
Payments for legal settlements									$ 4,000
Settlement, further distribution of default interchange									$ 500
Loss contingency, further distributions, default period									8 months
Loss contingency, number of cases appointed counsel | litigation_case		2
Europe Merchant Litigation
Loss Contingencies [Line Items]
Number of separate cases | case_filed				2
Canadian Competition Proceedings
Loss Contingencies [Line Items]
Number of financial institutions | financial_institution						10
Number of provinces with cases | province								5
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
U.S. Covered Litigation | Interchange Multidistrict Litigation
Loss Contingencies [Line Items]
Provision for legal matters			$ 1,100
Settled Litigation | Canadian Competition Proceedings
Loss Contingencies [Line Items]
Number of financial institutions | entity										4
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]
Period of accrual	5 years
Loss contingency accrual							1,198	$ 1,428					$ 978
Possible return to defendants	$ 467
U.S. Covered Litigation | Interchange Multidistrict Litigation
Loss Contingencies [Line Items]
Provision for legal matters							$ 370	$ 600
U.S. Covered Litigation | Visa, MasterCard, and Certain U.S. Financial Institutions | Interchange Multidistrict Litigation
Loss Contingencies [Line Items]
Provision for legal matters	$ 900
Subsequent Event | Interchange Opt Out Litigation
Loss Contingencies [Line Items]
Number of opt-out cases filed | case_filed												50
Subsequent Event | Europe Merchant Litigation
Loss Contingencies [Line Items]
Number of plaintiffs | merchant											500
Loss Contingency, Claims Settled, Number | merchant											100
Number of claims pending | merchant											400	400
Subsequent Event | Threatened Litigation | Europe Merchant Litigation
Loss Contingencies [Line Items]
Number of plaintiffs | merchant											30