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Financial Derivative Instruments Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Derivative [Line Items]        
Interest Rate Swap Contract Length     9 years  
Premiums paid for fuel derivative contracts $ 28 $ 34 $ 55 $ 67
Percentage of actual consumption hedged 78.00%   81.00%  
Current Unrealized Net Losses in OCI $ 15   $ 15  
Maximum sum of derivatives of counterparty to be included in other (less than $10 million) $ 10   $ 10  
Maximum cash collateral provided as a percentage of derivative contract value (in hundredths) 100.00%   100.00%  
Maximum letter of credit percent of collateral 100.00%   100.00%  
Remainder of Current Year        
Derivative [Line Items]        
Percent of fuel consumption hedged 63.00%   63.00%