DEBT - Debt Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Less debt issuance costs	$ (46)	$ (15)
Total debt	4,933	2,471
Less current portion	442	289
Long-term debt, less current portion	$ 4,491	$ 2,182
Weighted-average fixed-interest rate	3.90%	3.40%
Weighted-average variable-interest rate	6.30%	6.80%
Fixed-rate notes payable due through 2029
Debt Instrument [Line Items]
Long-term debt, including adjustments	$ 56	$ 80
Fixed-rate PSP notes payable due through 2031
Debt Instrument [Line Items]
Long-term debt, including adjustments	688	600
Fixed-rate EETCs payable due through 2027
Debt Instrument [Line Items]
Long-term debt, including adjustments	864	835
Fixed-rate Japanese Yen denominated notes payable due through 2031
Debt Instrument [Line Items]
Long-term debt, including adjustments	88	0
Variable-rate notes payable due through 2036
Debt Instrument [Line Items]
Long-term debt, including adjustments	1,283	971
Loyalty financing, variable-rate term loan facility due through 2031
Debt Instrument [Line Items]
Long-term debt, including adjustments	750	0
Loyalty financing, fixed-rate notes due through 2029 and 2031
Debt Instrument [Line Items]
Long-term debt, including adjustments	$ 1,250	$ 0