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Revenue - Air Traffic Liability Rollforward (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Air Traffic Liability Roll Forward    
ATL, beginning balance $ 5,070 $ 4,565
Current Period Sales 16,410 16,200
Revenue amounts in beginning balance (3,160) (3,098)
Revenue from Current Period Sales (12,354) (12,084)
ATL, ending balance $ 5,966 $ 5,583