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Contingencies and Legal Proceedings Legal Proceedings (Details)
3 Months Ended
Dec. 31, 2020
USD ($)
Units
Agreements
Groups
Legal Proceedings  
Number of licensed nuclear units | Units 7
Legal loss contingency accrual $ 12,000,000
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