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Financial Derivative Instruments Narrative (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Derivative [Line Items]    
Derivative, Cost of Hedge $ 34 $ 34
Percentage of actual consumption hedged 84.00%  
Current Unrealized Net Gains (Losses) in OCI $ 17  
Maximum sum of derivatives of counterparty to be included in other (less than $9 million) $ 9  
Maximum cash collateral provided as a percentage of derivative contract value (in hundredths) 100.00%  
Maximum letter of credit percent of collateral 100.00%  
Remainder of Current Year    
Derivative [Line Items]    
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage 76.00%