LONG-TERM DEBT - SCHEDULE OF LONG-TERM DEBT (Details) - USD ($) $ in Millions	12 Months Ended			21 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2021
Debt Instrument [Line Items]
Long-term Debt	$ 2,159.0	$ 2,539.0		$ 2,539.0
Less debt issuance costs	(15.0)	(20.0)		(20.0)
Less current portion	276.0	366.0		366.0
Long-term debt, net of current portion	$ 1,883.0	$ 2,173.0		$ 2,173.0
Weighted-average fixed-interest rate	3.50%	3.70%		3.70%
Weighted-average variable-interest rate	5.80%	1.30%		1.30%
Proceeds from issuance of long-term debt, net of issuance costs	$ 0.0	$ 363.0	$ 2,564.0
Long-term debt payments	385.0	1,334.0	565.0
Long-term Debt, Fiscal Year Maturity [Abstract]
2023	280.0
2024	244.0
2025	296.0
2026	176.0
2027	535.0
Thereafter	643.0
Long-term Debt	2,174.0
Unsecured Term Loan through PSP of CARES Act	$ 600.0	600.0		$ 600.0
Adjustments to Additional Paid in Capital, Warrant Issued		16.0	14.0
Long-term Debt, Weighted Average Interest Rate, at Point in Time	3.70%
Adjustments to Additional Paid in Capital, Warrant Issued		16.0	$ 14.0
Scheduled debt payments [Member]
Debt Instrument [Line Items]
Repayments of debt	$ 368.0
Debt prepayments [Member]
Debt Instrument [Line Items]
Repayments of debt	17.0
Fixed-rate notes payable due through 2029
Debt Instrument [Line Items]
Long-term Debt	113.0	163.0		163.0
Variable-rate notes payable due through 2029
Debt Instrument [Line Items]
Long-term Debt		738.0		738.0
Variable Rate Notes Payable due through 2029
Debt Instrument [Line Items]
Long-term Debt	514.0
July 2020 EETC
Debt Instrument [Line Items]
Long-term Debt	$ 947.0	$ 1,058.0		1,058.0
US Treasury CARES Act Loan
Long-term Debt, Fiscal Year Maturity [Abstract]
Adjustments to Additional Paid in Capital, Warrant Issued				30.0
Adjustments to Additional Paid in Capital, Warrant Issued				$ 30.0