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Contingencies and Legal Proceedings Contingencies (Details)
$ in Millions
 9 Months Ended
Jun. 30, 2018
USD ($)
reactors
Insurance_layers
Procedures
Sep. 30, 2017
USD ($)
Loss Contingencies    
Nuclear liability insurance $ 450  
Assessment from licensees for each licensed reactor $ 127  
Number of licensed reactors in US | reactors 99  
Nuclear accident assessment limitation per year per unit $ 19  
Maximum assessment per nuclear incident 891  
Maximum payment required per accident in any one year 133  
Total amount of protection available $ 13,000  
Surcharge for legal expenses 5.00%  
Number of layers before Congress is required to take action | Insurance_layers 2  
Amount of property, decommissioning, and decontamination insurance carried $ 5,100  
Amount of insurance available for loss at any one site 2,100  
Maximum amount of retrospective premiums 128  
Maximum idemnity if a covered accident takes or keeps a nuclear unit offline 490  
Maximum retrospective premiums 44  
Estimated future decommissioning cost $ 4,402 $ 4,304
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 $ 197  
Possible additional future costs for compliance with CCR requirements 1,200  
Possible additional future costs for compliance with Clean Water requirements. 480  
Estimated liability for cleanup and environmental work 10 7
Nuclear    
Loss Contingencies    
Estimated future decommissioning cost 2,956 2,859
Non-nuclear    
Loss Contingencies    
Estimated future decommissioning cost $ 1,446 $ 1,445