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Credit Risks (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Credit Derivatives [Line Items]    
Letters of Credit Outstanding, Amount $ 107  
Energy commodity derivative contracts(a)    
Credit Derivatives [Line Items]    
Letters of Credit Outstanding, Amount 0  
Initial Margin Requirements 13  
Variation Margin Requirements 12  
Contract and Over the Counter [Member] | Energy commodity derivative contracts(a)    
Credit Derivatives [Line Items]    
Derivative, Collateral, Right to Reclaim Cash 1 $ 37
One notch credit downgrade [Member] | Energy commodity derivative contracts(a)    
Credit Derivatives [Line Items]    
Additional Collateral, Aggregate Fair Value 31  
Two notch credit downgrade [Member] | Contract and Over the Counter [Member] | Energy commodity derivative contracts(a)    
Credit Derivatives [Line Items]    
Derivative, Collateral, Right to Reclaim Cash $ 0