DEBT - Debt Obligations (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Debt Instrument [Line Items]
Less debt issuance costs and other	$ (44)	$ (46)
Total debt	5,009	4,933
Less current portion	519	442
Long-term debt, net of current portion	$ 4,490	$ 4,491
Weighted-average fixed-interest rate	3.90%	3.90%
Weighted-average variable-interest rate	5.90%	6.30%
Fixed-rate notes payable due through 2037
Debt Instrument [Line Items]
Total debt, gross	$ 121	$ 56
Fixed-rate PSP notes payable due through 2031
Debt Instrument [Line Items]
Total debt, gross	630	688
Fixed-rate EETCs payable due through 2027
Debt Instrument [Line Items]
Total debt, gross	714	864
Fixed-rate Japanese Yen denominated notes payable due through 2031
Debt Instrument [Line Items]
Total debt, gross	56	88
Variable-rate PSP notes payable due through 2030
Debt Instrument [Line Items]
Total debt, gross	62	0
Variable-rate notes payable due through 2037
Debt Instrument [Line Items]
Total debt, gross	1,476	1,283
Loyalty financing, variable-rate term loan facility due through 2031
Debt Instrument [Line Items]
Total debt, gross	744	750
Loyalty financing, fixed-rate notes due through 2031
Debt Instrument [Line Items]
Total debt, gross	$ 1,250	$ 1,250