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Asset Retirement Obligations Asset Retirement Obligations (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Liabilities Settled [Line Items]      
Increase in ARO liability $ 217    
Change in estimate [1] 203 $ 1,077  
Allen West ARO increase   273  
Gallatin CCR ARO increase (decrease)   173  
Additional obligations [1] 43    
Balance [1] 7,002 6,785 $ 5,616
Asset Retirement Obligation, Liabilities Settled [1] 242 114  
Accretion (recorded as regulatory asset) [1] 213 206  
Asset Retirement Obligation, Current 266 345  
Amortization and Depreciation of Decontaminating and Decommissioning Assets 72 169 144
Total ARO change due to CCR 122 129  
Change in ARO from operating CCR facilities 30 38  
Change in ARO from inactive CCR facilities 13 91  
Increase in CCR post-closure care   460  
Change in plant decommissioning obligations   19  
ARO change due to groundwater monitoring 69    
Nuclear      
Liabilities Settled [Line Items]      
Change in estimate 12 0  
Additional obligations 0    
Balance 3,428 3,278 3,136
Asset Retirement Obligation, Liabilities Settled 11 1  
Accretion (recorded as regulatory asset) 149 143  
Non-nuclear      
Liabilities Settled [Line Items]      
Change in estimate 191 1,077  
Additional obligations 43    
Balance 3,574 3,507 $ 2,480
Asset Retirement Obligation, Liabilities Settled 231 113  
Accretion (recorded as regulatory asset) $ 64 $ 63  
[1]
(1) Includes $266 million and $345 million at September 30, 2021 and 2020, respectively, in Current liabilities.

TVA implemented revised depreciation rates during the first quarter of 2022 applicable to its completed plant as a result of the completion of a new depreciation study. The study includes a decline in the service life estimates of TVA’s coal-fired plants based on current planning assumptions to potentially retire the remainder of the coal-fired fleet by 2035. As a result of the accelerated retirements reflected in the depreciation study, TVA performed an assessment of the assumptions used in the timing of cash flows related to its non-nuclear AROs. Based on the assessment, TVA identified changes to its projections of timing of certain asset retirement processes, that will be recorded in 2022.