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Commitments and Contingencies - Contingencies (Details)
$ in Millions
 12 Months Ended
Sep. 30, 2022
USD ($)
Insurance_layers
reactors
Units
Procedures
Sep. 30, 2021
USD ($)
Sep. 30, 2020
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 96    
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 $ 13,700    
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 115    
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] $ 7,162 $ 7,002 $ 6,785
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 16 17 19
Possible additional future costs for compliance with Clean Air Act requirements 128    
Possible additional future costs for compliance with CCR requirements 900    
Possible additional future costs for compliance with Clean Water requirements. 111    
Estimated liability for cleanup and similar environmental work on a non-discounted basis 17 18  
Amount of insurance available for loss at any one site, max 2,800    
Nuclear      
Contingencies      
Estimated future decommissioning cost 3,643 3,428 3,278
Non-nuclear      
Contingencies      
Estimated future decommissioning cost $ 3,519 $ 3,574 $ 3,507
[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.