REVENUE - Loyalty Program Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Disaggregation of Revenue [Line Items]
Total Operating Revenue	$ 4,308	$ 3,766	$ 11,673	$ 10,607
Loyalty Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	650	558	1,870	1,652
Passenger | Loyalty Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	387	358	1,122	1,035
Loyalty program other | Loyalty Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	$ 263	$ 200	$ 748	$ 617