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Revenue - Air Traffic Liability Rollforward (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Air Traffic Liability Rollforward [Abstract]    
ATL, beginning balance $ 4,565 $ 4,221
Current Period Sales 21,026 20,146
Revenue amounts in beginning balance (3,479) (3,099)
Revenue from Current Period Sales (17,042) (16,703)
ATL, ending balance $ 5,070 $ 4,565