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Asset Retirement Obligations Asset Retirement Obligations (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Liabilities Settled [Line Items]      
Increase in ARO liability $ 327    
Change in estimate [1] 369 $ 247  
Balance [1] 7,489 7,162 $ 7,002
Asset Retirement Obligation, Liabilities Settled [1] 279 311  
Accretion (recorded as regulatory asset) [1] 237 224  
Asset Retirement Obligation, Current 272 275  
Amortization and Depreciation of Decontaminating and Decommissioning Assets 188 137 72
Total ARO change due to CCR 458 82  
Change due to cost study   58  
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 60 30  
Change in cost estimates at Allen   15  
Changes in closure liabilities 15    
Increase in Asset Retirement Obligation, Revisions in Estimate 9    
Nuclear      
Liabilities Settled [Line Items]      
Change in estimate 7 61  
Balance 3,808 3,643 3,428
Asset Retirement Obligation, Liabilities Settled 8 2  
Accretion (recorded as regulatory asset) 166 156  
Non-nuclear      
Liabilities Settled [Line Items]      
Change in estimate 362 186  
Balance 3,681 3,519 $ 3,574
Asset Retirement Obligation, Liabilities Settled 271 309  
Accretion (recorded as regulatory asset) $ 71 $ 68  
[1] (1) Includes $272 million and $275 million at September 30, 2023 and 2022, respectively, in Current liabilities.