Legal Matters - Accrued Litigation (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	$ 914	$ 1,203
Reestablishment of prior accrual related to interchange multidistrict litigation	0	467
Balance at end of period	983	914
Uncovered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for unsettled legal matters	4	10
Covered Litigation
Loss Contingency Accrual [Roll Forward]
Provision for unsettled legal matters	125	26
Reestablishment of prior accrual related to interchange multidistrict litigation	0	467
Payments on unsettled and settled matters	(60)	(792)
U.S. Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	888	1,198
Reestablishment of prior accrual related to interchange multidistrict litigation	0	467
Payments on unsettled and settled matters	(7)	(777)
Balance at end of period	881	888
VE Territory Covered Litigation
Loss Contingency Accrual [Roll Forward]
Balance at beginning of period	21	5
Provision for unsettled legal matters	125	26
Payments on unsettled and settled matters	(44)	(10)
Balance at end of period	$ 102	$ 21

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=121557415&loc=d3e14326-108349

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Amount of cash outflow reducing loss contingency liability.

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Amount charged against operating income increasing loss contingency liability, after adjustments to reduce previously estimated charges.

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Return Of Takedown Payment To The Litigation Escrow Account

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