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Revenue - Air Traffic Liability Rollforward (Details) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Air Traffic Liability Roll Forward    
ATL, beginning balance $ 4,565 $ 4,221
Current Period Sales 10,927 10,853
Revenue amounts in beginning balance [1] (2,613) (2,370)
Revenue from Current Period Sales [1] (7,349) (7,562)
ATL, ending balance $ 5,530 $ 5,142
[1] (a) Does not include certain ancillary revenues that are purchased on the day of travel, which do not flow through Air traffic liability.