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Contingencies and Legal Proceedings Legal Proceedings (Details)
 3 Months Ended
Dec. 31, 2019
USD ($)
megawatts
Units
Groups
Agreements
Sep. 30, 2019
USD ($)
Legal Proceedings    
Number of licensed nuclear units | Units 7  
Amount paid for remediation under amended consent decree   $ 100,000
General    
Legal Proceedings    
Legal loss contingency accrual $ 13,000,000  
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  
Number of units to be idled | Units 18  
Megawatts | megawatts 2,200  
Megawatts option 2 3,500  
Amount to be invested in certain TVA environmental projects $ 290,000,000  
Amount to be provided to fund environmental projects 60,000,000  
Amount to pay civil penalties $ 10,000,000