Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Income Tax Disclosure [Abstract]
Balance at beginning of the year	$ 22	$ 20	$ 159
Additions based on tax positions related to the current year	2	1	1
Additions for tax positions of prior years	8	3
Reductions of tax positions of prior years	(2)	(1)	(137)
Reductions related to settlements	(1)	(1)	(2)
Reductions from lapse of statute of limitations			(1)
Balance at end of the year	$ 29	$ 22	$ 20

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Amount of unrecognized tax benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 217
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482663/740-10-55-217

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 15A
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-15A

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 10B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482525/740-10-45-10B

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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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
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-Paragraph 15A
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-URI https://asc.fasb.org/1943274/2147482685/740-10-50-15A

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