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Contingencies and Legal Proceedings Legal Proceedings (Details)	9 Months Ended
	Jun. 30, 2021 USD ($) Agreements Groups Units
Legal Proceedings
Number of licensed nuclear units | Units	7
Amount remaining to be spent under environmental agreements [Abstract]	$ 11,000,000
General
Legal Proceedings
Legal loss contingency accrual	$ 14,000,000
Environmental Agreements
Legal Proceedings
Number of similar environmental agreements entered into | Agreements	2
Number of environmental agreements entered into with the EPA | Agreements	1
Number of environmental agreements entered into with environmental advocacy groups | Groups	3
Amount to be invested in certain TVA environmental projects	$ 290,000,000
Accounts payable and accrued liabilities | General
Legal Proceedings
Legal loss contingency accrual	$ 30,000,000

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

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