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Regulatory Assets and Liabilities Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Regulatory Assets and Liabilities      
Non-current regulatory liabilities $ 23 $ 0  
Accelerated Amortization of Deferred Nuclear Generating Units and Training Costs     $ 857
Accelerated recovery for Kingston ash spill 266    
Current regulatory assets 130 156  
Regulatory assets 10,245 8,763  
Regulatory Assets 10,375 8,919  
Current regulatory liabilities 141 150  
Regulatory asset amount expensed 23 261 $ 2
Gallatin CCR   672  
Regulatory Liabilities 164 150  
Reclass Environmental agreements to Other reg assets   12  
Deferred Project Costs [Member]      
Regulatory Assets and Liabilities      
Gallatin coal combustion residual facilities estimated cost to cap and close 114 89  
Regulatory Asset      
Regulatory Assets and Liabilities      
Regulatory assets 138 142  
Removal Costs [Member]      
Regulatory Assets and Liabilities      
Regulatory assets 1,506 1,241  
Unrealized losses on interest rate derivatives      
Regulatory Assets and Liabilities      
Current regulatory assets 4 39  
Regulatory assets 0 15  
Non-nuclear decommissioning costs      
Regulatory Assets and Liabilities      
Regulatory assets 2,512 1,741  
Pension Costs [Member]      
Regulatory Assets and Liabilities      
Regulatory assets 5,193 4,756  
Deferred Fuel Costs [Member]      
Regulatory Assets and Liabilities      
Current regulatory assets 12 28  
Nuclear decommissioning costs      
Regulatory Assets and Liabilities      
Regulatory assets 896 868  
Unrealized gains/losses on commodity derivatives      
Regulatory Assets and Liabilities      
Non-current regulatory liabilities 23 0  
Current regulatory liabilities 26 12  
Fuel cost adjustment tax equivalents      
Regulatory Assets and Liabilities      
Current regulatory liabilities $ 115 $ 138