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Revenue Local Power Company Contracts (Details)
 3 Months Ended
Dec. 31, 2019
USD ($)
percent
number_of_local_power_companies
Dec. 31, 2018
USD ($)
Long-term Contract for Purchase of Electric Power [Line Items]    
Total number of customers 154  
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 2,578,000,000 $ 2,725,000,000
Total number of customers that represent the percent of sales | percent 1  
Total long-duration contracts revenue recognition 2,357,000,000  
5-year Termination Notice [Member]    
Long-term Contract for Purchase of Electric Power [Line Items]    
Number of customers | number_of_local_power_companies 16  
Number of customers that represent the percent of sales | percent 0  
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 672,000,000  
10-year Termination Notice [Member]    
Long-term Contract for Purchase of Electric Power [Line Items]    
Number of customers | number_of_local_power_companies 4  
Number of customers that represent the percent of sales | percent 0  
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 214,000,000  
20-year Termination Notice [Member]    
Long-term Contract for Purchase of Electric Power [Line Items]    
Number of customers | number_of_local_power_companies 134  
Number of customers that represent the percent of sales | percent 1  
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 1,471,000,000