Legal Matters Legal Matters - Accrued Litigation (Details) $ in Millions	1 Months Ended	12 Months Ended		36 Months Ended
	Dec. 31, 2010 financial_institution	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)
Loss Contingency Accrual [Roll Forward]
Accrued Litigation, Beginning of Period		$ 1,024	$ 1,456
Accrued Litigation, End of Period		981	1,024	$ 981
Unsettled
Loss Contingency Accrual [Roll Forward]
Provision for unsettled legal matters		2	14
Settled Litigation
Loss Contingency Accrual [Roll Forward]
Payments on unsettled and settled matters		(47)	(446)
U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Accrued Litigation, Beginning of Period		1,023	1,449
Accrued Litigation, End of Period		978	1,023	978
U.S. Covered Litigation | Settled Litigation
Loss Contingency Accrual [Roll Forward]
Payments on unsettled and settled matters		(45)	(426)
Covered Litigation | Settled Litigation
Loss Contingency Accrual [Roll Forward]
Payments on unsettled and settled matters				(528)
U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Accrued Litigation, Beginning of Period		0
Provision for unsettled legal matters		2
Accrued Litigation, End of Period		$ 2	$ 0	$ 2
Canadian Competition Proceedings
Loss Contingencies [Line Items]
Number of Financial Institutions | financial_institution	10