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Risk Management Activities and Derivative Transactions Counterparty Credit Risk (Details)
$ in Millions
3 Months Ended 9 Months Ended
Jun. 30, 2020
USD ($)
megawatts
Jun. 30, 2020
USD ($)
megawatts
Jun. 30, 2019
Sep. 30, 2019
USD ($)
Derivative        
Accounts Receivable, before Allowance for Credit Loss, Current $ 1,286 $ 1,286   $ 1,624
Total derivatives not subject to master netting or similar arrangement $ 10 $ 10   $ 12
Two Largest Customer Percentage of Total Operating Revenue 1800.00% 1700.00% 1600.00%  
Moody's, A1 Rating [Member]        
Derivative        
Banking Counterparties Credit Rating   A3    
Natural Gas Banking Counterparties Credit Rating   A2    
Moody's, B1 Rating [Member]        
Derivative        
Coal Banking Counterparties Credit Rating   B1    
Moody's, Ba1 Rating [Member]        
Derivative        
Coal Banking Counterparties Credit Rating   Ba1    
Moody's, Caa1 Rating [Member]        
Derivative        
Natural Gas Banking Counterparties Credit Rating   Caa2    
Long-term Contract for Purchase of Electric Power [Domain]        
Derivative        
Megawatts | megawatts 440 440