LONG-TERM DEBT - SCHEDULE OF LONG-TERM DEBT (Details) - USD ($) $ in Millions	12 Months Ended			60 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Apr. 22, 2030	Feb. 05, 2021	Jan. 15, 2021
Debt Instrument [Line Items]
Long-term Debt	$ 3,495.0	$ 1,499.0
Less debt issuance costs and unamortized debt discount	(33.0)	(8.0)
Less current portion	1,138.0	235.0
Long-term debt, net of current portion	$ 2,357.0	$ 1,264.0
Weighted-average fixed-interest rate	4.30%	3.30%
Weighted-average variable-interest rate	1.90%	2.90%
Proceeds from issuance of long-term debt, net of issuance costs	$ 2,564.0	$ 450.0	$ 339.0
Long-term debt payments	565.0	1,058.0	$ 807.0
Repayments of debt	314.0
Long-term Debt, Fiscal Year Maturity [Abstract]
2021	1,145.0
2022	371.0
2023	334.0
2024	240.0
2025	396.0
Thereafter	1,042.0
Long-term Debt	3,528.0
Unsecured Term Loan through PSP of CARES Act	290.0	0.0
Adjustments to Additional Paid in Capital, Warrant Issued	$ 14.0
Long-term Debt, Weighted Average Interest Rate, at Point in Time	3.30%
Adjustments to Additional Paid in Capital, Warrant Issued	$ 14.0
Subsequent Event
Long-term Debt, Fiscal Year Maturity [Abstract]
Unsecured Term Loan through PSP of CARES Act					$ 130.0	$ 51.0
Aircraft Type [Domain]
Long-term Debt, Fiscal Year Maturity [Abstract]
Debt Instrument, Collateral	32
Debt Instrument, Collateral	32
Fixed-rate notes payable due through 2029
Debt Instrument [Line Items]
Long-term Debt	$ 198.0	475.0
Variable-rate notes payable due through 2029
Debt Instrument [Line Items]
Long-term Debt		1,032.0
Variable Rate Notes Payable due through 2029
Debt Instrument [Line Items]
Long-term Debt	1,866.0
Long-term Debt, Fiscal Year Maturity [Abstract]
Proceeds from Issuance of Long-term Debt	589.0
Proceeds from Issuance of Long-term Debt	589.0
July 2020 EETC
Debt Instrument [Line Items]
Long-term Debt	$ 1,174.0	$ 0.0
July 2020 EETC | B737 | Aircraft Type [Domain]
Long-term Debt, Fiscal Year Maturity [Abstract]
Debt Instrument, Collateral	42
Debt Instrument, Collateral	42
July 2020 EETC | E175 [Member] | Aircraft Type [Domain]
Long-term Debt, Fiscal Year Maturity [Abstract]
Debt Instrument, Collateral	19
Debt Instrument, Collateral	19
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
US Treasury CARES Act Loan | Aircraft Type [Domain]
Long-term Debt, Fiscal Year Maturity [Abstract]
Debt Instrument, Collateral	34
Debt Instrument, Collateral	34
US Treasury CARES Act Loan | 3724 Aircraft Engines and Engine Parts
Long-term Debt, Fiscal Year Maturity [Abstract]
Debt Instrument, Collateral	15
Debt Instrument, Collateral	15
CARES Act Unsecured Interest Rate
Long-term Debt, Fiscal Year Maturity [Abstract]
Debt Instrument, Interest Rate, Stated Percentage	1.00%
Debt Instrument, Interest Rate, Stated Percentage	1.00%
CARES Act Unsecured Interest Rate | Subsequent Event
Long-term Debt, Fiscal Year Maturity [Abstract]
Debt Instrument, Basis Spread on Variable Rate				2.00%
Debt Instrument, Basis Spread on Variable Rate				2.00%