REVENUE RECOGNITION - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenue from Contract with Customer [Abstract]
Revenue recognized that was previously recorded in air traffic liability	$ 7,400	$ 4,200	$ 2,200
Disaggregation of Revenue [Line Items]
Marketing contracts initial terms, minimum (in years)	1 year
Marketing contracts initial terms, maximum (in years)	13 years
Cash sales of miles from marketing agreements	$ 6,900	5,700	$ 4,100
Majority of new miles, redemption period (in years)	2 years
Air traffic
Disaggregation of Revenue [Line Items]
Deferred revenue (current and noncurrent)		8,300
Loyalty program deferred revenue		$ 100