Reconciliation of Beginning and Ending Unrecognized Tax Benefits by Fiscal Year (Detail) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning Balance	$ 1,353	$ 1,160
Increases of unrecognized tax benefits related to prior years	367	56
Decreases of unrecognized tax benefits related to prior years	(233)	(59)
Increases of unrecognized tax benefits related to current year	172	197
Reductions to unrecognized tax benefits related to lapsing statute of limitations	(1)	(1)
Ending Balance	$ 1,658	$ 1,353

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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- Definition

Amount of unrecognized tax benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 45
-Paragraph 10B
-URI http://asc.fasb.org/extlink&oid=84176650&loc=SL37586934-109318

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-SubTopic 10
-Section 50
-Paragraph 15A
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- Definition

Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 15A
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- Definition

Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

+ References

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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

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