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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Collateral (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Sep. 30, 2017
Derivative    
Likely collateral obligation increase if downgraded $ 22  
Collateralized Securities [Member]    
Derivative    
Aggregate fair value of derivative instruments with credit-risk related contingent features that were in a liability position 1,300  
Collateral obligations 900  
Letter of Credit    
Derivative    
Amount of letters of credit outstanding $ 900 $ 1,200