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Revenue - Air Traffic Liability Rollforward (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Change in Contract With Customer, Liability [Roll Forward]    
Beginning balance $ 7,133 $ 5,510
Current period sales (passenger travel, ancillary services, flight loyalty, and partner loyalty) 10,648 7,631
Revenue from amounts included in contract liability opening balances (2,346) (2,126)
Revenue from current period sales (7,199) 3,921
Ending balance $ 8,236 $ 7,094