Legal Matters - Schedule of Accrued Litigation for Both Covered and Non-Covered Litigation (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	$ 914	$ 1,203
Balance at end of period	909	1,629
U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	888	1,198
Balance at end of period	881	1,624
VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	21	5
Provision for legal matters	10	1
Balance at end of period	22	5
Uncovered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for legal matters	1	0
Covered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for legal matters	10	1
Interchange Multidistrict Litigation
Loss Contingency Accrual [Roll Forward]
Reestablishment of prior accrual related to interchange multidistrict litigation	0	467
Interchange Multidistrict Litigation | U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for legal matters	0	467
Settled Litigation
Loss Contingency Accrual [Roll Forward]
Payments for legal matters	(16)	(42)
Settled Litigation | U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Payments for legal matters	(7)	(41)
Settled Litigation | VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Payments for legal matters	$ (9)	$ (1)