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Contingencies and Legal Proceedings Contingencies (Details)
$ in Millions
 6 Months Ended
Mar. 31, 2019
USD ($)
Insurance_layers
Procedures
reactors
Sep. 30, 2018
USD ($)
Loss Contingencies    
Nuclear liability insurance $ 450  
Assessment from licensees for each licensed reactor $ 138  
Number of licensed reactors in US | reactors 99  
Nuclear accident assessment limitation per year per unit $ 20  
Maximum assessment per nuclear incident 963  
Total amount of protection available $ 14,100  
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 $ 4,977 $ 4,779
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 $ 153  
Possible additional future costs for compliance with CCR requirements 1,155  
Possible additional future costs for compliance with Clean Water requirements. 313  
Estimated liability for cleanup and environmental work 16 12
Nuclear    
Loss Contingencies    
Estimated future decommissioning cost 3,070 2,989
Non-nuclear    
Loss Contingencies    
Estimated future decommissioning cost $ 1,907 $ 1,790