REVENUE - Loyalty Program Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Disaggregation of Revenue [Line Items]
Total Operating Revenue	$ 3,766	$ 3,072	$ 10,607	$ 8,201
Loyalty Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	558	460	1,652	1,336
Passenger | Loyalty Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	358	289	1,035	827
Loyalty program other | Loyalty Plan Revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	$ 200	$ 171	$ 617	$ 509