REVENUE - Loyalty Program Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Disaggregation of Revenue [Line Items]
Total Operating Revenue	$ 4,065	$ 3,704	$ 7,365	$ 6,841
Loyalty Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	646	571	1,220	1,094
Passenger | Loyalty Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	388	361	735	677
Loyalty program other | Loyalty Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	$ 258	$ 210	$ 485	$ 417