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Commitments and Contingencies Commitments and Contingencies - Contingencies (Details)
$ in Millions
 12 Months Ended
Sep. 30, 2019
USD ($)
Units
Insurance_layers
Procedures
reactors
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Contingencies      
Loss Contingency, Damages Sought, Value $ 30    
Nuclear liability insurance 450    
Assessment from licensees for each licensed reactor $ 138    
Number of licensed reactors in US | reactors 98    
Nuclear accident assessment limitation per year per unit $ 20    
Number of licensed nuclear units | Units 7    
Maximum assessment per nuclear incident $ 963    
Total amount of protection available $ 14,000    
Number of layers until the U.S. Congress is required to take action | Insurance_layers 2    
Amount of insurance available for loss at any one site $ 2,100    
Maximum amount of retrospective premiums 139    
Maximum idemnity if a covered accident tasks or keeps a nuclear unit offline 490    
Maximum amount of retrospective premiums 46    
Estimated future decommissioning cost [1] $ 5,616 $ 4,779 $ 4,304
Number of procedures for determining estimates for the costs of nuclear decommissioning | Procedures 2    
Amount spent to reduce emissions since 1970 $ 6,800    
Amount spent to reduce emissions 17 62 206
Possible additional future costs for compliance with Clean Air Act requirements 142    
Possible additional future costs for compliance with CCR requirements 1,181    
Possible additional future costs for compliance with Clean Water requirements. 278    
Estimated liability for cleanup and similar environmental work on a non-discounted basis 15 12  
Nuclear      
Contingencies      
Estimated future decommissioning cost 3,136 2,989 2,859
Non-nuclear      
Contingencies      
Estimated future decommissioning cost $ 2,480 $ 1,790 $ 1,445
[1] (1) The current portions of the ARO liability in the amounts of $163 million and $115 million at September 30, 2019 and 2018, respectively, are included in Accounts payable and accrued liabilities.