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REVENUE RECOGNITION - Narrative (Details) - USD ($)
$ in Billions
6 Months Ended
Jun. 30, 2024
Jun. 30, 2023
Revenue from Contract with Customer [Abstract]    
Revenue recognized that was previously deferred $ 5.6 $ 5.7
Cash sales of mileage credits $ 3.6 $ 3.4
Redemption period for majority of new miles (in years) 2 years