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Regulatory Assets and Liabilities Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2018
Sep. 30, 2016
Sep. 30, 2017
Regulatory Assets and Liabilities      
Accelerated Amortization of Deferred Nuclear Generating Units and Training Costs $ 857    
Current regulatory assets 414   $ 447
Regulatory Assets 7,026   9,145
Yearly amortization of Deferred nuclear generating units 237    
Additional Accelerated Amortization, Deferred Nuclear Generating Units 764    
Disposal Group, Including Discontinued Operation, Consideration   $ 111  
Proceeds from Sale of Other Property, Plant, and Equipment   $ 22  
Proceeds Due at Closing $ 89    
Max Years to Close 2 years    
Period collected in rates 15 years    
Additional Accelerated Amortization, Nuclear Training Costs $ 93    
Environmental Agreements      
Regulatory Assets and Liabilities      
Initial regulatory asset 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 $ 276    
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    
Other current regulatory assets      
Regulatory Assets and Liabilities      
Current regulatory assets 0   2
Deferred Nuclear Generating Units and Construction Costs      
Regulatory Assets and Liabilities      
Current regulatory assets $ 0   $ 237