LONG-TERM DEBT - SCHEDULE OF LONG-TERM DEBT (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term Debt	$ 2,539.0	$ 3,495.0
Less debt issuance costs and unamortized debt discount	(20.0)	(33.0)
Less current portion	366.0	1,138.0
Long-term debt, net of current portion	$ 2,173.0	$ 2,357.0
Weighted-average fixed-interest rate	3.70%	4.30%
Weighted-average variable-interest rate	1.30%	1.90%
Proceeds from issuance of long-term debt, net of issuance costs	$ 363.0	$ 2,564.0	$ 450.0
Long-term debt payments	1,334.0	565.0	$ 1,058.0
Repayments of debt	923.0
Long-term Debt, Fiscal Year Maturity [Abstract]
2022	371.0
2023	334.0
2024	240.0
2025	261.0
2026	176.0
Thereafter	1,177.0
Long-term Debt	2,559.0
Unsecured Term Loan through PSP of CARES Act	600.0	290.0
Adjustments to Additional Paid in Capital, Warrant Issued	$ 16.0
Long-term Debt, Weighted Average Interest Rate, at Point in Time	3.30%
Adjustments to Additional Paid in Capital, Warrant Issued	$ 16.0
Fixed-rate notes payable due through 2029
Debt Instrument [Line Items]
Long-term Debt	163.0	198.0
Variable-rate notes payable due through 2029
Debt Instrument [Line Items]
Long-term Debt		1,866.0
Variable Rate Notes Payable due through 2029
Debt Instrument [Line Items]
Long-term Debt	738.0
Long-term Debt, Fiscal Year Maturity [Abstract]
Proceeds from Issuance of Long-term Debt	54.0
Proceeds from Issuance of Long-term Debt	54.0
July 2020 EETC
Debt Instrument [Line Items]
Long-term Debt	1,058.0	1,174.0
US Treasury CARES Act Loan
Long-term Debt, Fiscal Year Maturity [Abstract]
Adjustments to Additional Paid in Capital, Warrant Issued		6.0
Adjustments to Additional Paid in Capital, Warrant Issued		$ 6.0
CARES Act PSP Term Loan
Long-term Debt, Fiscal Year Maturity [Abstract]
Proceeds from Issuance of Long-term Debt	311.0
Proceeds from Issuance of Long-term Debt	$ 311.0