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Derivatives and Risk Management (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Fuel Derivative                
Derivative                
Expected proceeds from derivative instruments       $ 300   $ 300    
Expected payments for derivative instruments               $ 300
Scenario, Forecast | Fuel Derivative                
Derivative                
Expected payments for derivative instruments             $ 300  
Fuel hedge contracts                
Derivative                
Derivative gains (losses) $ 40 $ (41) $ 97   $ (315)      
Fuel hedge contracts | Airline                
Derivative                
Settlement of derivative contract   $ 455