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Financial Derivative Instruments Narrative (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Derivative [Line Items]        
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage 58.00%   58.00%  
Premiums paid for fuel derivative contracts $ 34 $ 33 $ 117 $ 81
Current Unrealized Net Gains (Losses) in OCI (401)   (401)  
Maximum sum of derivatives of counterparty to be included in other (less than $20 million) $ 2   $ 2  
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%  
Cash Flow Hedging        
Derivative [Line Items]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net [1] $ (143) (82) $ (491) (175)
Cash Flow Hedging | Fuel derivatives        
Derivative [Line Items]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net [1],[2] (141) (79) (484) (166)
Cash Flow Hedging | Interest rate derivatives        
Derivative [Line Items]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net [1],[2] $ (2) $ (3) $ (7) $ (9)
Remainder of Current Year        
Derivative [Line Items]        
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage 72.00%   72.00%  
[1] Amounts related to fuel derivative contracts and interest rate derivatives, which are included in Fuel and oil and Interest expense, respectively.
[2] Net of tax