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Asset Retirement Obligations Asset Retirement Obligations (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2020
Liabilities Settled [Line Items]      
Increase in ARO liability $ 160    
Change in estimate [1] 247 $ 203  
Additional obligations [1]   43  
Balance [1] 7,162 7,002 $ 6,785
Asset Retirement Obligation, Liabilities Settled [1] 311 242  
Accretion (recorded as regulatory asset) [1] 224 213  
Asset Retirement Obligation, Current 275 266  
Amortization and Depreciation of Decontaminating and Decommissioning Assets 137 72 169
Total ARO change due to CCR 82 122  
Change in ARO from operating CCR facilities   30  
Change in ARO from inactive CCR facilities   13  
ARO change due to groundwater monitoring   69  
Accounts payable and accrued liabilities      
Liabilities Settled [Line Items]      
Asset Retirement Obligation, Liabilities Settled 20    
Nuclear      
Liabilities Settled [Line Items]      
Change in estimate 61 12  
Additional obligations   0  
Balance 3,643 3,428 3,278
Asset Retirement Obligation, Liabilities Settled 2 11  
Accretion (recorded as regulatory asset) 156 149  
Change due to cost study 58    
Non-nuclear      
Liabilities Settled [Line Items]      
Change in estimate 186 191  
Additional obligations   43  
Balance 3,519 3,574 $ 3,507
Asset Retirement Obligation, Liabilities Settled 309 231  
Accretion (recorded as regulatory asset) 68 $ 64  
Change in estimate due to coal fines 119    
Change in estimate due to closure of coal yards 57    
Change in estimate due to timing of maintenance 53    
Change in estimate due to timing of asset retirement activities 47    
Change in estimate due to Gallatin CCR closure 30    
Change in cost estimates at Allen $ 15    
[1]
(1) Includes $275 million and $266 million at September 30, 2022 and 2021, respectively, in Current liabilities.
(2) Settlements include the change in asset retirement obligation project accruals included in Accounts payable and accrued liabilities of $20 million.