Legal Matters Legal Matters - Accrued Litigation (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 27, 2014	Sep. 30, 2018	Sep. 30, 2017
Loss Contingency Accrual [Roll Forward]
Accrued Litigation, Beginning of Period		$ 982	$ 981
Accrued Litigation, End of Period		1,434	982
Settled Litigation
Loss Contingency Accrual [Roll Forward]
Payments on unsettled and settled matters		(156)	(204)
Covered Litigation [Member]
Loss Contingency Accrual [Roll Forward]
Provision for unsettled legal matters		601
Uncovered Litigation [Member]
Loss Contingency Accrual [Roll Forward]
Provision for unsettled legal matters		7	19
U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Accrued Litigation, Beginning of Period		978	978
Accrued Litigation, End of Period		1,428	978
U.S. Covered Litigation | Settled Litigation
Loss Contingency Accrual [Roll Forward]
Payments on unsettled and settled matters		(150)	0
VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Accrued Litigation, Beginning of Period		1	2
Provision for unsettled legal matters		1	186
Accrued Litigation, End of Period		0	1
VE Territory Covered Litigation | Settled Litigation
Loss Contingency Accrual [Roll Forward]
Payments on unsettled and settled matters		(2)	(187)
Interchange Multidistrict Litigation | U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for unsettled legal matters	$ 1,100
Interchange Multidistrict Litigation | U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for unsettled legal matters		$ 600	$ 0