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Risk Management Credit Risks (Details) - Energy commodity derivative contracts(a) - USD ($)
Jun. 30, 2016
Dec. 31, 2015
Credit Derivatives [Line Items]    
Letters of Credit Outstanding, Amount $ 0 $ 2,000,000
Cash collateral held(b)    
Credit Derivatives [Line Items]    
Collateral posted(c) 18,000,000 0
Derivative, Collateral, Obligation to Return Cash 0 $ (37,000,000)
One notch credit downgrade [Member]    
Credit Derivatives [Line Items]    
Additional Collateral, Aggregate Fair Value 0  
Two notch credit downgrade [Member]    
Credit Derivatives [Line Items]    
Additional Collateral, Aggregate Fair Value $ 0