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Risk Management Activities and Derivative Transactions Collateral (Details) - USD ($)
$ in Millions
Jun. 30, 2020
Sep. 30, 2019
Derivative    
Forward Contract Derivative Asset, at Fair Value $ 11 $ 22
Likely cash collateral obligation increase 22  
Collateralized Securities [Member]    
Derivative    
Derivative, Net Liability Position, Aggregate Fair Value 2,400  
Derivative, Collateral, Right to Reclaim Cash 1,500  
Letter of Credit    
Derivative    
Amount of letters of credit outstanding 1,510 1,300
Fair Value, Inputs, Level 2    
Derivative    
Forward Contract Derivative Asset, at Fair Value $ 11 $ 22