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Revenue Local Power Company Contracts (Details)
$ in Millions
 3 Months Ended
Dec. 31, 2020
USD ($)
Units
Dec. 31, 2019
USD ($)
Long-term Contract for Purchase of Electric Power [Line Items]    
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 2,304 $ 2,578
Total long-duration contracts revenue recognition 2,091  
Total percentage of operating revenues 90.80%  
Number of LPCs signed LTA | Units 142  
Total Number of LPCs | Units 153  
Percent of op revenues of LPCs who filed complaint with FERC 400.00%  
5-year Termination Notice [Member]    
Long-term Contract for Purchase of Electric Power [Line Items]    
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 371  
Percentage of total operating revenues 16.10%  
Number of LPCs signed LTA | Units 11  
20-year Termination Notice [Member]    
Long-term Contract for Purchase of Electric Power [Line Items]    
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 1,720  
Percentage of total operating revenues 74.70%