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Revenue Local Power Company Contracts (Details)
$ in Millions
 3 Months Ended 9 Months Ended
Jun. 30, 2020
USD ($)
Units
percent
Jun. 30, 2019
USD ($)
Jun. 30, 2020
USD ($)
Units
percent
Jun. 30, 2019
USD ($)
Long-term Contract for Purchase of Electric Power [Line Items]        
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 2,251 $ 2,604 $ 7,350 $ 8,079
Total long-duration contracts revenue recognition 2,058   6,716  
Total percentage of operating revenues 91.40%   91.40%  
Number of LPCs signed LTA | Units 141   141  
Total Number of LPCs | Units 154   154  
5-year Termination Notice [Member]        
Long-term Contract for Purchase of Electric Power [Line Items]        
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 427   $ 1,316  
Percentage of total operating revenues 19.00%   17.90%  
Number of LPCs signed LTA | Units 12   12  
10-year Termination Notice [Member]        
Long-term Contract for Purchase of Electric Power [Line Items]        
Number of customers that represent the percent of sales | percent 0   0  
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 1   $ 1  
Number of LPCs signed LTA | Units 1   1  
20-year Termination Notice [Member]        
Long-term Contract for Purchase of Electric Power [Line Items]        
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 1,630   $ 5,399  
Percentage of total operating revenues 72.40%   73.50%