Legal Matters - Schedule of Accrued Litigation for Both Covered and Non-Covered Litigation (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	$ 1,203	$ 1,434
Provision for legal matters	1	90
Balance at end of period	1,629	1,489
U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	1,198	1,428
Balance at end of period	1,624	1,428
VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	5	0
Provision for legal matters	1	90
Balance at end of period	5	55
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	0	7
Settled Litigation
Loss Contingency Accrual [Roll Forward]
Payments for legal matters	(42)	(42)
Settled Litigation | U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Payments for legal matters	(41)	0
Settled Litigation | VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Payments for legal matters	$ (1)	$ (35)