Legal Matters - Schedule of Accrued Litigation for Both Covered and Non-Covered Litigation (Detail) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	$ 1,203	$ 1,434	$ 1,434
Provision for legal matters	7	159
Balance at end of period	1,268	914	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	1,251	828	1,198
VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	5	0	0
Provision for legal matters	7	159
Balance at end of period	11	61	$ 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	35
Settled Litigation
Loss Contingency Accrual [Roll Forward]
Payments for legal matters	(416)	(714)
Settled Litigation | U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Payments for legal matters	(414)	(600)
Settled Litigation | VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Payments for legal matters	$ (1)	$ (98)