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Risk Management Credit Risks (Details) - Energy commodity derivative contracts
Jun. 30, 2018
USD ($)
notches
Dec. 31, 2017
USD ($)
Credit Derivatives [Line Items]    
Letters of Credit Outstanding, Amount $ 0 $ 0
Initial Margin Requirements 10,000,000  
Variation Margin Requirements $ 13,000,000  
One Notch Downgrade to Credit Rating | notches 1  
Two Notch Downgrade to Credit Rating | notches 2  
Cash collateral held    
Credit Derivatives [Line Items]    
Collateral posted(b)   $ 1,000,000
One notch credit downgrade [Member]    
Credit Derivatives [Line Items]    
Additional Collateral, Aggregate Fair Value $ 100,000,000  
Two notch credit downgrade [Member]    
Credit Derivatives [Line Items]    
Additional Collateral, Aggregate Fair Value 9,000,000  
Restricted Deposit [Member] | Cash collateral held    
Credit Derivatives [Line Items]    
Collateral posted(b) $ 23,000,000