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Revenue Local Power Company Contracts (Details)
 3 Months Ended 6 Months Ended
Mar. 31, 2019
USD ($)
Customers
percent
Mar. 31, 2018
USD ($)
Mar. 31, 2019
USD ($)
Customers
percent
Mar. 31, 2018
USD ($)
Long-term Contract for Purchase of Electric Power [Line Items]        
Total number of customers 154   154  
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 2,750,000,000 $ 2,792,000,000 $ 5,475,000,000 $ 5,341,000,000
Total number of customers that represent the percent of sales | percent 1   1  
Total long-duration contracts revenue recognition 2,514,000,000   4,981,000,000  
5-year Termination Notice [Member]        
Long-term Contract for Purchase of Electric Power [Line Items]        
Number of customers | Customers 86   86  
Number of customers that represent the percent of sales | percent 1   1  
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 1,472,000,000      
6-year Termination Notice [Member]        
Long-term Contract for Purchase of Electric Power [Line Items]        
Number of customers | Customers 1   1  
Number of customers that represent the percent of sales | percent 0   0  
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 13,000,000      
10-year Termination Notice [Member]        
Long-term Contract for Purchase of Electric Power [Line Items]        
Number of customers | Customers 52   52  
Number of customers that represent the percent of sales | percent 0   0  
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 862,000,000      
12-year Termination Notice [Member]        
Long-term Contract for Purchase of Electric Power [Line Items]        
Number of customers | Customers 1   1  
Number of customers that represent the percent of sales | percent 0   0  
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 6,000,000      
15-year Termination Notice [Member]        
Long-term Contract for Purchase of Electric Power [Line Items]        
Number of customers | Customers 11   11  
Number of customers that represent the percent of sales | percent 0   0  
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 127,000,000      
20-year Termination Notice [Member]        
Long-term Contract for Purchase of Electric Power [Line Items]        
Number of customers | Customers 3   3  
Number of customers that represent the percent of sales | percent 0   0  
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block] $ 34,000,000