Legal Matters - Schedule of Accrued Litigation for Both Covered and Non-Covered Litigation (Detail) - USD ($) $ in Millions	9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	$ 914	$ 1,203
Balance at end of period	900	1,156
Uncovered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for legal matters	5	7
Covered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for legal matters	23	14
Reestablishment of prior accrual related to interchange multidistrict litigation	0	467
Payments for legal matters	(42)	(535)
U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	888	1,198
Reestablishment of prior accrual related to interchange multidistrict litigation	0	467
Payments for legal matters	(7)	(529)
Balance at end of period	881	1,136
VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	21	5
Provision for legal matters	23	14
Payments for legal matters	(29)	(5)
Balance at end of period	$ 15	$ 14