REVENUE - Passenger 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
Passenger revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	3,355	2,651	6,163	4,655
Passenger revenue | Domestic
Disaggregation of Revenue [Line Items]
Total Operating Revenue	3,061	2,490	5,523	4,295
Passenger revenue | Latin America
Disaggregation of Revenue [Line Items]
Total Operating Revenue	166	161	380	360
Passenger revenue | Pacific
Disaggregation of Revenue [Line Items]
Total Operating Revenue	128	0	260	0
Passenger ticket revenue, net of taxes and fees | Passenger revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	2,832	2,226	5,184	3,874
Passenger ancillary revenue | Passenger revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	162	135	302	243
Loyalty program passenger revenue | Passenger revenue
Disaggregation of Revenue [Line Items]
Total Operating Revenue	$ 361	$ 290	$ 677	$ 538