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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% |  |

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