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Asset Retirement Obligations Asset Retirement Obligations (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Aug. 01, 2017
Liabilities Settled [Line Items]      
Increase in ARO liability $ 837    
Change in estimate [1] 50 $ 430  
Costs for asbestos 114    
Allen West ARO increase 33    
Paradise ARO increase (decrease) 57    
Allen East ARO increase (decrease) 44    
Gallatin CCR ARO increase (decrease) 672    
Additional obligations [1] 18 1  
Gallatin CCR [1] 672    
Gallatin CCR regulatory asset     $ 900
Balance [1] 5,616 4,779  
Asset Retirement Obligation, Liabilities Settled [1] 89 106  
Accretion (recorded as regulatory asset) [1] 186 165  
Asset disposition [1]   (15)  
Gain (Loss) on Disposition of Assets 5    
Amortization and Depreciation of Decontaminating and Decommissioning Assets 144    
Accounts payable and accrued liabilities      
Liabilities Settled [Line Items]      
Current portion of ARO 163 115  
Nuclear      
Liabilities Settled [Line Items]      
Revision of estimate (other non nuclear) 0 (250)  
Change in estimate   0  
Additional obligations 18 0  
Gallatin CCR 0    
Balance 3,136 2,989  
Asset Retirement Obligation, Liabilities Settled 7 0  
Accretion (recorded as regulatory asset) 136 130  
Asset disposition   0  
Non-nuclear      
Liabilities Settled [Line Items]      
Change in estimate 50 430  
Additional obligations 0 1  
Gallatin CCR 672    
Balance 2,480 1,790  
Asset Retirement Obligation, Liabilities Settled 82 106  
Accretion (recorded as regulatory asset) $ 50 35  
Asset disposition   $ (15)  
[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.