Legal Matters - Schedule of Accrued Litigation for Both Covered and Non-Covered Litigation (Detail) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	$ 914	$ 1,203
Balance at end of period	886	1,268
Uncovered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for legal matters	3	7
Covered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for legal matters	9	7
Reestablishment of prior accrual related to interchange multidistrict litigation	0	467
Payments for legal matters	(40)	(416)
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)	(414)
Balance at end of period	881	1,251
VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	21	5
Provision for legal matters	9	7
Payments for legal matters	(28)	(1)
Balance at end of period	$ 2	$ 11