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Revenue - Air Traffic Liability - Loyalty Program Rollforward (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Change in Contract With Customer, Liability [Roll Forward]        
Beginning balance $ 4,719 $ 3,856 $ 4,447 $ 3,385
Amounts deferred associated with points awarded 688 447 1,810 1,488
Unused funds converted to loyalty points 0 105 0 105
Ending balance 4,772 4,187 4,772 4,187
Passenger Customer        
Change in Contract With Customer, Liability [Roll Forward]        
Revenue recognized from points redeemed (621) (205) (1,448) (747)
Other        
Change in Contract With Customer, Liability [Roll Forward]        
Revenue recognized from points redeemed $ (14) $ (16) $ (37) $ (44)