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Commitments and Contingencies - Contingencies (Details)
$ in Millions
 12 Months Ended
Sep. 30, 2021
USD ($)
Units
reactors
Procedures
Insurance_layers
Sep. 30, 2020
USD ($)
Sep. 30, 2019
USD ($)
Contingencies      
Loss Contingency, Damages Sought, Value $ 30    
Nuclear liability insurance 450    
Assessment from licensees for each licensed reactor $ 138    
Number of licensed reactors in US | reactors 95    
Nuclear accident assessment limitation per year per unit $ 20    
Number of licensed nuclear units | Units 7    
Maximum assessment per nuclear incident $ 963    
Total amount of protection available $ 13,500    
Number of layers until the U.S. Congress is required to take action | Insurance_layers 2    
Amount of insurance available for loss at any one site $ 2,100    
Maximum amount of retrospective premiums 128    
Maximum idemnity if a covered accident tasks or keeps a nuclear unit offline 490    
Maximum amount of retrospective premiums 43    
Estimated future decommissioning cost [1] $ 7,002 $ 6,785 $ 5,616
Number of procedures for determining estimates for the costs of nuclear decommissioning | Procedures 2    
Amount spent to reduce emissions since 1970 $ 6,800    
Amount spent to reduce emissions 17 19 17
Possible additional future costs for compliance with Clean Air Act requirements 159    
Possible additional future costs for compliance with CCR requirements 789    
Possible additional future costs for compliance with Clean Water requirements. 148    
Estimated liability for cleanup and similar environmental work on a non-discounted basis 18 14  
Amount of insurance available for loss at any one site, max 2,800    
Nuclear      
Contingencies      
Estimated future decommissioning cost 3,428 3,278 3,136
Non-nuclear      
Contingencies      
Estimated future decommissioning cost $ 3,574 $ 3,507 $ 2,480
[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.