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Commitments and Contingencies - Contingencies (Details)
$ in Millions
 12 Months Ended
Sep. 30, 2025
USD ($)
Procedures
Insurance_layers
reactors
Units
Sep. 30, 2024
USD ($)
Sep. 30, 2023
USD ($)
Contingencies      
Nuclear liability insurance $ 500    
Assessment from licensees for each licensed reactor $ 166    
Number of licensed reactors in US | reactors 95    
Nuclear accident assessment limitation per year per unit $ 25    
Number of licensed nuclear units | Units 7    
Maximum assessment per nuclear incident $ 1,200    
Total amount of protection available $ 16,300    
Number of layers until the U.S. Congress is required to take action | Insurance_layers 2    
Maximum amount of retrospective premiums $ 122    
Maximum idemnity if a covered accident tasks or keeps a nuclear unit offline 490    
Maximum amount of retrospective premiums 50    
Estimated future decommissioning cost [1] $ 10,414 $ 10,806 $ 7,489
Number of procedures for determining estimates for the costs of nuclear decommissioning | Procedures 2    
Estimated liability for cleanup and similar environmental work on a non-discounted basis $ 8 $ 15  
Amount of insurance available for loss at any one site (bottom range) 2,100    
Amount of insurance available for loss at any one site (top range) $ 2,800    
Environmental Remediation      
Contingencies      
Accrual for Environmental Loss Contingencies, Significant Assumptions 319 million 215 million  
Nuclear      
Contingencies      
Estimated future decommissioning cost $ 3,976 $ 3,814 3,808
Non-nuclear      
Contingencies      
Estimated future decommissioning cost $ 6,438 $ 6,992 $ 3,681
[1]
(1) Includes $313 million and $283 million at September 30, 2025 and 2024, respectively, in Current liabilities.