Legal Matters - Schedule of Accrued Litigation for Both Covered and Non-Covered Litigation (Detail) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	$ 1,203	$ 1,434	$ 1,434
Provision for legal matters	14	165
Balance at end of period	1,156	856	1,203
U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	1,198	1,428	1,428
Balance at end of period	1,136	828	1,198
VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	5	0	0
Provision for legal matters	14	165
Balance at end of period	14	9	$ 5
Interchange Multidistrict Litigation
Loss Contingency Accrual [Roll Forward]
Provision for legal matters	467	0
Interchange Multidistrict Litigation | U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for legal matters	467	0
Unsettled
Loss Contingency Accrual [Roll Forward]
Provision for legal matters	7	37
Settled Litigation
Loss Contingency Accrual [Roll Forward]
Payments for legal matters	(535)	(780)
Settled Litigation | U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Payments for legal matters	(529)	(600)
Settled Litigation | VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Payments for legal matters	$ (5)	$ (156)