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Asset Retirement Obligations Asset Retirement Obligations (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Liabilities Settled [Line Items]      
Increase in ARO liability $ 1,200    
Change in estimate [1] 1,077 $ 50  
Costs for asbestos   114  
Allen West ARO increase 273 33  
Paradise ARO increase (decrease)   57  
Allen East ARO increase (decrease)   44  
Gallatin CCR ARO increase (decrease) 173 672  
Additional obligations [1]   18  
Gallatin CCR   672  
Balance [1] 6,785 5,616 $ 4,779
Asset Retirement Obligation, Liabilities Settled [1] 114 89  
Accretion (recorded as regulatory asset) [1] 206 186  
Asset Retirement Obligation, Current 345 163  
Amortization and Depreciation of Decontaminating and Decommissioning Assets 169 144 144
Total ARO change due to CCR 129    
Change in ARO from operating CCR facilities 38    
Change in ARO from inactive CCR facilities 91    
Increase in CCR post-closure care 460    
Change in plant decommissioning obligations 19    
Accounts payable and accrued liabilities      
Liabilities Settled [Line Items]      
Asset Retirement Obligation, Current 345 163  
Nuclear      
Liabilities Settled [Line Items]      
Revision of estimate (other non nuclear) 0 (250)  
Change in estimate   0  
Additional obligations   18  
Gallatin CCR   0  
Balance 3,278 3,136 2,989
Asset Retirement Obligation, Liabilities Settled 1 7  
Accretion (recorded as regulatory asset) 143 136  
Non-nuclear      
Liabilities Settled [Line Items]      
Change in estimate 1,077 50  
Additional obligations   0  
Gallatin CCR   672  
Balance 3,507 2,480 $ 1,790
Asset Retirement Obligation, Liabilities Settled 113 82  
Accretion (recorded as regulatory asset) $ 63 $ 50  
[1] (1) Includes $345 million and $163 million at September 30, 2020 and 2019, respectively, in Current liabilities.