Income Taxes - Reconciliation of Beginning and Ending Unrecognized Tax Benefits by Fiscal Year (Detail) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of period	$ 1,658	$ 1,353	$ 1,160
Increases of unrecognized tax benefits related to prior years	216	367	56
Decreases of unrecognized tax benefits related to prior years	(13)	(233)	(59)
Increases of unrecognized tax benefits related to current year	384	172	197
Decreases related to settlements with taxing authorities	(9)	0	0
Reductions related to lapsing statute of limitations	(2)	(1)	(1)
Balance at end of period	$ 2,234	$ 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

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

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Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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

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

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

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