DEBT - Debt Obligations (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Debt Instrument [Line Items]
Less debt issuance costs	$ (52)	$ (45)
Total debt	5,976	5,309
Less current portion	763	540
Long-term debt, net of current portion	$ 5,213	$ 4,769
Weighted-average fixed-interest rate	5.30%	3.90%
Weighted-average variable-interest rate	5.50%	5.20%
Fixed-rate notes payable due through 2038
Debt Instrument [Line Items]
Total debt, gross	$ 204	$ 115
Fixed-rate PSP notes payable due through 2031
Debt Instrument [Line Items]
Total debt, gross	0	630
Fixed-rate EETCs payable due through 2027
Debt Instrument [Line Items]
Total debt, gross	512	715
Fixed-rate Japanese Yen denominated notes payable due through 2031
Debt Instrument [Line Items]
Total debt, gross	43	51
Fixed-rate unsecured notes payable due through 2031
Debt Instrument [Line Items]
Total debt, gross	500	0
Variable-rate Japanese Yen denominated notes payable due through 2033
Debt Instrument [Line Items]
Total debt, gross	136	149
Variable-rate PSP notes payable due through 2031
Debt Instrument [Line Items]
Total debt, gross	684	62
Variable-rate notes payable due through 2038
Debt Instrument [Line Items]
Total debt, gross	1,463	1,639
Loyalty financing, variable-rate term loan facility due through 2033
Debt Instrument [Line Items]
Total debt, gross	1,236	743
Loyalty financing, fixed-rate notes due through 2031
Debt Instrument [Line Items]
Total debt, gross	$ 1,250	$ 1,250