REVENUE - Loyalty Program Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Disaggregation of Revenue [Line Items]
Total Operating Revenue	$ 3,704	$ 2,897	$ 6,841	$ 5,129
Mileage Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	571	464	1,094	876
Passenger | Mileage Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	361	290	677	538
Mileage plan other revenue | Mileage Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	$ 210	$ 174	$ 417	$ 338