LONG-TERM DEBT - SCHEDULE OF LONG-TERM DEBT (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended			21 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2021
Debt Instrument [Line Items]
Long-term Debt	$ 2,471	$ 2,159
Less debt issuance costs	(15)	(15)
Less current portion	289	276
Long-term debt, net of current portion	$ 2,182	$ 1,883
Weighted-average fixed-interest rate	3.40%	3.50%
Weighted-average variable-interest rate	6.80%	5.80%
Proceeds from issuance of long-term debt, net of issuance costs	$ 411	$ 0	$ 363
Long-term debt payments	282	385	1,334
Long-term Debt, Fiscal Year Maturity [Abstract]
2024	294
2025	351
2026	307
2027	630
2028	114
Thereafter	790
Long-term Debt	2,486
Proceeds from Issuance of Long-term Debt	595
Unsecured Term Loan through PSP of CARES Act	$ 600	600
Adjustments to Additional Paid in Capital, Warrant Issued			16
Long-term Debt, Weighted Average Interest Rate, at Point in Time	4.40%
Proceeds from Issuance of Long-term Debt	$ 595
Adjustments to Additional Paid in Capital, Warrant Issued			$ 16
Class of Warrant or Right, Exercise Price of Warrants or Rights	$ 39.06
Scheduled debt payments [Member]
Debt Instrument [Line Items]
Long-term debt payments	$ 282
Fixed-rate notes payable due through 2029
Debt Instrument [Line Items]
Long-term Debt	80	113
Variable-rate notes payable due through 2035
Debt Instrument [Line Items]
Long-term Debt	971	514
Fixed-rate EETC payable due through 2027 [Member]
Debt Instrument [Line Items]
Long-term Debt	835	$ 947
US Treasury CARES Act Loan
Long-term Debt, Fiscal Year Maturity [Abstract]
Adjustments to Additional Paid in Capital, Warrant Issued				$ 30
Adjustments to Additional Paid in Capital, Warrant Issued				$ 30
Proceeds from Issuance of Long-Term Debt, Noncash proceeds
Long-term Debt, Fiscal Year Maturity [Abstract]
Proceeds from Issuance of Long-term Debt	179
Proceeds from Issuance of Long-term Debt	179
Proceeds from Issuance of Long-Term Debt, Cash proceeds
Long-term Debt, Fiscal Year Maturity [Abstract]
Proceeds from Issuance of Long-term Debt	416
Proceeds from Issuance of Long-term Debt	$ 416