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Contingencies and Legal Proceedings Contingencies (Details)
$ in Millions
 6 Months Ended 12 Months Ended
Mar. 31, 2025
USD ($)
Insurance_layers
Procedures
reactors
Sep. 30, 2024
USD ($)
Loss 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  
Maximum assessment per nuclear incident 1,200  
Total amount of protection available 16,300  
Amount of insurance available for loss at any one site 2,100  
Maximum amount of retrospective premiums 115  
Maximum idemnity if a covered accident takes or keeps a nuclear unit offline 490  
Maximum retrospective premiums 44  
Estimated future decommissioning cost $ 10,878 $ 10,806
Number of procedures for determining estimates for the costs of nuclear decommissioning | Procedures 2  
Estimated liability for cleanup and environmental work $ 10 15
Amount of insurance available for loss at any one site (top range) 2,800  
Long-term debt of VIEs, net (including current maturities) $ 1,702 $ 966
The U.S. Congress is required to take action if these layes are exhausted | Insurance_layers 2  
Environmental Remediation    
Loss Contingencies    
Accrual for Environmental Loss Contingencies, Significant Assumptions 276 million 215 million
Accrual for Environmental Loss Contingencies, Significant Assumptions 276 million 215 million
Nuclear    
Loss Contingencies    
Estimated future decommissioning cost $ 3,893 $ 3,814
Non-nuclear    
Loss Contingencies    
Estimated future decommissioning cost $ 6,985 $ 6,992