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Commitments and Contingencies - Contingencies (Details)
$ in Millions
 12 Months Ended
Sep. 30, 2023
USD ($)
reactors
Units
Procedures
Insurance_layers
Sep. 30, 2022
USD ($)
Sep. 30, 2021
USD ($)
Contingencies      
Nuclear liability insurance $ 450    
Assessment from licensees for each licensed reactor $ 138    
Number of licensed reactors in US | reactors 97    
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 $ 13,800    
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 114    
Maximum idemnity if a covered accident tasks or keeps a nuclear unit offline 490    
Maximum amount of retrospective premiums 44    
Estimated future decommissioning cost [1] $ 7,489 $ 7,162 $ 7,002
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 25 16 17
Possible additional future costs for compliance with Clean Air Act requirements 252    
Possible additional future costs for compliance with CCR requirements 1,600    
Possible additional future costs for compliance with Clean Water requirements. 190    
Estimated liability for cleanup and similar environmental work on a non-discounted basis 16 17  
Amount of insurance available for loss at any one site, max 2,800    
Nuclear      
Contingencies      
Estimated future decommissioning cost 3,808 3,643 3,428
Non-nuclear      
Contingencies      
Estimated future decommissioning cost $ 3,681 $ 3,519 $ 3,574
[1] (1) Includes $272 million and $275 million at September 30, 2023 and 2022, respectively, in Current liabilities.