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Financial Derivative Instruments Narrative (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Derivative [Line Items]    
Interest Rate Swap Contract Length 9 years  
Premiums paid for fuel derivative contracts $ 28 $ 34
Percentage of actual consumption hedged 84.00%  
Current Unrealized Net Gains in OCI $ 12  
Maximum sum of derivatives of counterparty to be included in other (less than $10 million) $ 10  
Maximum cash collateral provided as a percentage of derivative contract value (in hundredths) 100.00%  
Maximum letter of credit percent of collateral 100.00%  
Interest rate swap    
Derivative [Line Items]    
Interest Rate Derivative 12  
Remainder of Current Year    
Derivative [Line Items]    
Percent of fuel consumption hedged 67.00%