Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Income Tax Disclosure [Abstract]
Balance at beginning of the year	$ 30	$ 30	$ 41
Additions based on tax provisions related to the current year	2	2	1
Additions for tax positions of prior years	8	5
Reductions of tax provisions of prior years	(1)	(3)	(1)
Reductions related to settlements	(2)		(9)
Reductions from lapse of statute of limitations	(1)	(4)	(2)
Balance at end of the year	$ 36	$ 30	$ 30

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- Definition

Amount of unrecognized tax benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=SL6600010-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Unrecognized Tax Benefit
-URI http://asc.fasb.org/extlink&oid=6527854

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- Definition

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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