Legal Matters - Accrued Litigation (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018
Loss Contingency Accrual [Roll Forward]
Accrued Litigation, Beginning of Period	$ 1,434	$ 982
Accrued Litigation, End of Period	1,203	1,434
Settled Litigation
Loss Contingency Accrual [Roll Forward]
Payments on unsettled and settled matters	(803)	(156)
Uncovered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for unsettled legal matters	37	7
Covered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for unsettled legal matters	535	601
U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Accrued Litigation, Beginning of Period	1,428	978
Accrued Litigation, End of Period	1,198	1,428
U.S. Covered Litigation \| Settled Litigation
Loss Contingency Accrual [Roll Forward]
Payments on unsettled and settled matters	(600)	(150)
VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Accrued Litigation, Beginning of Period	0	1
Provision for unsettled legal matters	165	1
Accrued Litigation, End of Period	5	0
VE Territory Covered Litigation \| Settled Litigation
Loss Contingency Accrual [Roll Forward]
Payments on unsettled and settled matters	(160)	(2)
Interchange Multidistrict Litigation \| U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for unsettled legal matters	$ 370	$ 600

Amount of loss contingency liability.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942415&loc=d3e14326-108349

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