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Contingencies and Legal Proceedings Contingencies (Details)
 6 Months Ended
Mar. 31, 2021
USD ($)
Insurance_layers
Procedures
reactors
Sep. 30, 2020
USD ($)
Loss Contingencies    
Nuclear liability insurance $ 450,000,000  
Assessment from licensees for each licensed reactor $ 138,000,000  
Number of licensed reactors in US | reactors 96  
Nuclear accident assessment limitation per year per unit $ 20,000,000  
Maximum assessment per nuclear incident 963,000,000  
Total amount of protection available 13,700,000,000  
Amount of insurance available for loss at any one site 2,100,000,000  
Maximum amount of retrospective premiums 145,000,000  
Maximum idemnity if a covered accident takes or keeps a nuclear unit offline 490,000,000  
Maximum retrospective premiums 43,000,000  
Estimated future decommissioning cost $ 6,918,000,000 $ 6,785,000,000
Number of procedures for determining estimates for the costs of nuclear decommissioning | Procedures 2  
Possible additional future costs for compliance with Clean Air Act requirements $ 144,000,000  
Possible additional future costs for compliance with CCR requirements 860,000,000  
Possible additional future costs for compliance with Clean Water requirements. 160,000,000  
Estimated liability for cleanup and environmental work 15,000,000 14,000,000
Amount of insurance available for loss at any one site (top range) 2,800,000,000  
Long-term debt of VIEs (including current maturities), net 1,310,000,000 1,419,000,000
Legal loss contingency accrual $ 12,000,000  
The U.S. Congress is required to take action if these layes are exhausted | Insurance_layers 2  
Nuclear    
Loss Contingencies    
Estimated future decommissioning cost $ 3,351,000,000 3,278,000,000
Non-nuclear    
Loss Contingencies    
Estimated future decommissioning cost 3,567,000,000 $ 3,507,000,000
Environmental Agreements    
Loss Contingencies    
Amount invested in certain environmental projects 280,000,000  
General    
Loss Contingencies    
Long-term debt of VIEs (including current maturities), net 12,000,000  
Legal loss contingency accrual 14,000,000  
Accounts Payable and Accrued Liabilities $ 2,000,000