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Regulatory Assets and Liabilities Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Aug. 01, 2017
Regulatory Assets and Liabilities        
Non-current regulatory liabilities $ 0 $ 104    
Accelerated Amortization of Deferred Nuclear Generating Units and Training Costs 857      
Accelerated recovery for Kingston ash spill 266      
Current regulatory assets 156 414    
Regulatory assets 8,763 6,612    
Regulatory Assets 8,919 7,026    
Current regulatory liabilities 150 187    
Regulatory asset amount expensed 261 2 $ 143  
Gallatin CCR [1] $ 672      
Gallatin CCR regulatory asset       $ 900
Period collected in rates 15 years      
Regulatory Liabilities $ 150 291    
Environmental Agreements        
Regulatory Assets and Liabilities        
Initial liability recorded 360      
Investment in environmental agreement projects 290      
Amount to be provided to fund environmental projects 60      
Amount to be paid in civil penalties $ 10      
Environmental cleanup costs - Kingston ash spill        
Regulatory Assets and Liabilities        
Current regulatory asset amortization period 12 months      
Energy Efficiency Projects | Environmental Agreements        
Regulatory Assets and Liabilities        
Regulatory asset amount expensed $ 279      
Paid to States | Environmental Agreements        
Regulatory Assets and Liabilities        
Regulatory asset amount expensed 60      
Civil Penalties | Environmental Agreements        
Regulatory Assets and Liabilities        
Regulatory asset amount expensed 10      
Deferred Project Costs [Member]        
Regulatory Assets and Liabilities        
Gallatin coal combustion residual facilities estimated cost to cap and close 89 73    
Regulatory Asset        
Regulatory Assets and Liabilities        
Regulatory assets 130 118    
Removal Costs [Member]        
Regulatory Assets and Liabilities        
Regulatory assets 1,241 692    
Environmental Agreements        
Regulatory Assets and Liabilities        
Current regulatory assets 0 3    
Regulatory assets 12 11    
Unrealized losses on interest rate derivatives        
Regulatory Assets and Liabilities        
Current regulatory assets 39 4    
Regulatory assets 15 8    
Non-nuclear decommissioning costs        
Regulatory Assets and Liabilities        
Regulatory assets 1,741 1,019    
Pension Costs [Member]        
Regulatory Assets and Liabilities        
Regulatory assets 4,756 3,119    
Unrealized gains/losses on commodity derivatives        
Regulatory Assets and Liabilities        
Current regulatory assets 0 38    
Regulatory assets 0 861    
Deferred Fuel Costs [Member]        
Regulatory Assets and Liabilities        
Current regulatory assets 28 30    
Environmental cleanup costs - Kingston ash spill        
Regulatory Assets and Liabilities        
Current regulatory assets 0 266    
Nuclear decommissioning costs        
Regulatory Assets and Liabilities        
Regulatory assets 868 784    
Other current regulatory assets        
Regulatory Assets and Liabilities        
Gallatin CCR regulatory asset   38    
Regulatory Asset        
Regulatory Assets and Liabilities        
Gallatin CCR regulatory asset   861    
Unrealized gains/losses on commodity derivatives        
Regulatory Assets and Liabilities        
Non-current regulatory liabilities 0 31    
Current regulatory liabilities 12 41    
Deferred other post-retirement benefits cost        
Regulatory Assets and Liabilities        
Non-current regulatory liabilities 0 73    
Fuel cost adjustment tax equivalents        
Regulatory Assets and Liabilities        
Current regulatory liabilities $ 138 $ 146    
[1] (1) The current portions of the ARO liability in the amounts of $163 million and $115 million at September 30, 2019 and 2018, respectively, are included in Accounts payable and accrued liabilities.