REVENUE - Loyalty Program Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Disaggregation of Revenue [Line Items]
Total Operating Revenue	$ 3,300	$ 3,137
Loyalty Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	574	523
Passenger | Loyalty Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	347	316
Loyalty program other | Loyalty Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	$ 227	$ 207