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Revenue Recognition - Current And Non-Current Air Traffic Liability (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Revenue from Contract with Customer [Abstract]    
Beginning balance $ 733 $ 661
Increase (decrease) to air traffic liability - points redeemed (196) (153)
Increase (decrease) to air traffic liability - points earned 308 195
Ending balance $ 845 $ 703