Income Taxes - Summary of Gross Amounts of Unrecognized Tax Benefits (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 October 1	$ 543	$ 349	$ 469
Increase due to acquisitions	3	140	0
Increase due to current year tax positions	11	43	41
Increase due to prior year tax positions	6	43	19
Decreases due to prior year tax positions	(39)	0	(30)
Decrease due to settlements with tax authorities	0	(29)	(145)
Decrease due to lapse of statute of limitations	(5)	(3)	(5)
Balance at September 30	$ 519	$ 543	$ 349

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.

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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 lapses of applicable statutes of limitations.

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