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Revenue Local Power Company Contracts (Details)
$ in Millions
 12 Months Ended
Sep. 30, 2020
USD ($)
Units
Sep. 30, 2019
USD ($)
Sep. 30, 2018
USD ($)
Number of LPCs signed LTA | Units 142    
Electric revenue $ 10,104 $ 11,159 $ 11,075
Total number of LPCs | Units 153    
Percentage of total operating revenues 91.80%    
20-year contract arrangement [Member]      
Electric revenue $ 7,666    
5-year contract arrangement [Member]      
Electric revenue 1,740    
Local Power Company [Member]      
Electric revenue $ 9,406 $ 10,351 $ 10,262
5-year contract arrangement [Member]      
Number of LPCs signed LTA | Units 11    
Percentage of total operating revenues 17.00%