Benefit Plans - Pension Plans with Accumulated Benefit Obligations (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Retirement Benefits [Abstract]
Accumulated benefit obligation exceeds the fair value of plan assets, projected benefit obligation	$ 3,844	$ 3,623
Accumulated benefit obligation exceeds the fair value of plan assets, accumulated benefit obligation	3,703	3,476
Accumulated benefit obligation exceeds the fair value of plan assets, fair value of plan assets	2,936	2,821
Projected benefit obligation exceeds the fair value of plan assets, projected benefit obligation	3,920	3,698
Projected benefit obligation exceeds the fair value of plan assets, fair value of plan assets	$ 2,996	$ 2,882

No definition available.

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Amount of plan asset for defined benefit pension plan with projected benefit obligation in excess of plan assets.

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- Definition

Amount of projected benefit obligation for defined benefit pension plan with projected benefit obligation in excess of plan assets.

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- Definition

Amount of accumulated benefit obligation for defined benefit plan with accumulated benefit obligation in excess of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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Amount of plan asset for defined benefit plan with accumulated benefit obligation in excess of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of projected benefit obligation for defined benefit pension plan with accumulated benefit obligation in excess of plan assets.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2417-114920

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