DEBT - Debt Obligations (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Debt Instrument [Line Items]
Less debt issuance costs	$ (55)	$ (45)
Total debt	6,048	5,309
Less current portion	324	540
Long-term debt, net of current portion	$ 5,724	$ 4,769
Weighted-average fixed-interest rate	5.20%	3.90%
Weighted-average variable-interest rate	5.30%	5.20%
Fixed-rate notes payable due through 2038
Debt Instrument [Line Items]
Total debt, gross	$ 212	$ 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	549	715
Fixed-rate Japanese Yen denominated notes payable due through 2031
Debt Instrument [Line Items]
Total debt, gross	45	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	683	62
Variable-rate notes payable due through 2038
Debt Instrument [Line Items]
Total debt, gross	1,489	1,639
Loyalty financing, variable-rate term loan facility due through 2033
Debt Instrument [Line Items]
Total debt, gross	1,239	743
Loyalty financing, fixed-rate notes due through 2031
Debt Instrument [Line Items]
Total debt, gross	$ 1,250	$ 1,250