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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Collateral (Details) - USD ($)
$ in Millions
Sep. 30, 2016
Sep. 30, 2015
Derivative    
Securities Borrowed, Collateral, Obligation to Return Cash $ 0 $ 15
Likely collateral obligation increase if downgraded 22  
Collateral    
Derivative    
Aggregate fair value of derivative instruments with credit-risk related contingent features that were in a liability position 2,100  
Collateral obligations 1,400  
Letter of Credit    
Derivative    
Amount of letters of credit outstanding $ 1,400 $ 1,100