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Contingencies and Legal Proceedings Legal Proceedings (Details)
 6 Months Ended
Mar. 31, 2019
USD ($)
megawatts
Units
Groups
Agreements
Sep. 30, 2018
USD ($)
Jun. 30, 2013
USD ($)
Legal Proceedings      
Number of licensed nuclear units | Units 7    
Amount paid for remediation under the consent decree     $ 150,000
Amount paid for remediation under amended consent decree   $ 100,000  
Estimated Litigation Liability $ 30,000,000    
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    
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    
Other long-term liabilities | General      
Legal Proceedings      
Legal loss contingency accrual 12,000,000    
Accounts payable and accrued liabilities | General      
Legal Proceedings      
Legal loss contingency accrual $ 2,000,000