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Commitments and Contingencies Commitments and Contingencies - Contingencies (Details)
$ in Millions
 12 Months Ended
Sep. 30, 2018
USD ($)
Units
reactors
Insurance_layers
Procedures
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
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    
Number of licensed nuclear units | Units 7    
Maximum assessment per nuclear incident $ 891    
Total amount of protection available $ 13,000    
Number of layers until the U.S. 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 tasks or keeps a nuclear unit offline 490    
Maximum amount of retrospective premiums 44    
Estimated future decommissioning cost [1] $ 4,779 $ 4,304 $ 4,052
Number of procedures for determining estimates for the costs of nuclear decommissioning | Procedures 2    
Amount spent to reduce emissions since 1970 $ 6,700    
Amount spent to reduce emissions 62 206 259
Possible additional future costs for compliance with Clean Air Act requirements 163    
Possible additional future costs for compliance with CCR requirements 1,178    
Possible additional future costs for compliance with Clean Water requirements. 466    
Estimated liability for cleanup and similar environmental work on a non-discounted basis 12 7  
Nuclear      
Contingencies      
Estimated future decommissioning cost 2,989 2,859 2,492
Non-nuclear      
Contingencies      
Estimated future decommissioning cost $ 1,790 $ 1,445 $ 1,560
[1] (1) The current portions of the ARO liability in the amounts of $115 million and $128 million as of September 30, 2018 and 2017, respectively, are included in Accounts payable and accrued liabilities.