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Revenue Local Power Company Contracts (Details)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2021
USD ($)
Units
Mar. 31, 2020
USD ($)
Mar. 31, 2021
USD ($)
Units
Mar. 31, 2020
USD ($)
Long-term Contract for Purchase of Electric Power [Line Items]        
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 2,572 $ 2,521 $ 4,876 $ 5,099
Total long-duration contracts revenue recognition 2,337   4,428  
Total percentage of operating revenues 90.90%   90.80%  
Number of LPCs signed LTA | Units 142   142  
Total Number of LPCs | Units 153   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] $ 403   $ 774  
Percentage of total operating revenues 15.70%   15.90%  
Number of LPCs signed LTA | Units 11   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] $ 1,934   $ 3,654  
Percentage of total operating revenues 75.20%   74.90%