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Asset Retirement Obligations (Details) - USD ($) $ in Millions	9 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Sep. 30, 2023
Asset Retirement Obligations
Asset Retirement Obligation, Period Increase (Decrease)	$ 3,300
Amortization and Depreciation of Decontaminating and Decommissioning Assets	141
Asset Retirement Obligation, Beginning Balance	7,489
Settlements	(190)
Asset Retirement Obligation, Revision of Estimate	136
Asset Retirement Obligation, Liabilities Incurred	3,136
Accretion Expense, Including Asset Retirement Obligations	200
Asset Retirement Obligation, Ending Balance	10,771
Current portion of ARO	(283)		$ (272)
Revision in ARO estimate	(164)
Gallatin Fossil Plant
Asset Retirement Obligations
Change in closure liabilities		$ 231
Cumberland Fossil Plant
Asset Retirement Obligations
Change in closure liabilities	76
Nuclear
Asset Retirement Obligations
Asset Retirement Obligation, Beginning Balance	3,808
Settlements	(4)
Asset Retirement Obligation, Revision of Estimate	(161)
Asset Retirement Obligation, Liabilities Incurred		$ 0
Accretion Expense, Including Asset Retirement Obligations	128
Asset Retirement Obligation, Ending Balance	3,771
Non-nuclear
Asset Retirement Obligations
Asset Retirement Obligation, Beginning Balance	3,681
Settlements	(186)
Asset Retirement Obligation, Revision of Estimate	297
Asset Retirement Obligation, Liabilities Incurred	3,136
Accretion Expense, Including Asset Retirement Obligations	72
Asset Retirement Obligation, Ending Balance	$ 7,000

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Reconciliation of Asset Retirement Obligations [Line Items]

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- Definition

Amount of accretion expense, which includes, but is not limited to, accretion expense from asset retirement obligations, environmental remediation obligations, and other contingencies.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 410
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481879/410-20-45-1

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The current period expense charged against earnings on decontaminating and decommissioning of facilities used in the normal conduct of business and not intended for resale to allocate or recognize the cost of the assets over their useful lives.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

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The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 410
-SubTopic 20
-Name Accounting Standards Codification
-Section 25
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481999/410-20-25-4

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 410
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481850/410-20-50-1

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Current portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 410
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481850/410-20-50-1

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Amount of asset retirement obligations incurred during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 410
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481850/410-20-50-1

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Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 410
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481850/410-20-50-1

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Amount of increase (decrease) in asset retirement obligations.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 410
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481850/410-20-50-1

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Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 410
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(4)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481850/410-20-50-1

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