Income Taxes - Summary of Gross Amounts of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at October 1	$ 577	$ 601	$ 407
Increase due to acquisitions	1	3	140
Increase due to current year tax positions	35	11	43
Increase due to prior year tax positions	76	6	43
Decreases due to prior year tax positions	(49)	(39)	0
Decrease due to settlements with tax authorities	(4)	0	(29)
Decrease due to lapse of statute of limitations	(16)	(5)	(3)
Balance at September 30	$ 620	$ 577	$ 601

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 acquisitions.

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

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