Income Taxes - Summary of Gross Amounts of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at October 1	$ 267	$ 354	$ 611
Increase due to acquisitions	0	2	2
Increase due to current year tax positions	22	40	23
Increase due to prior year tax positions	33	60	5
Decreases due to prior year tax positions	(29)	0	(4)
Decrease due to settlements with tax authorities	(6)	(77)	(183)
Decrease due to lapse of statute of limitations	(18)	(112)	(100)
Balance at September 30	269	267	354
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	$ 366	$ 348	$ 447

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

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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 acquisitions.

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

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The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

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