Debt and Other Obligations - Schedule of Long-term Debt Instruments (Details) $ in Millions	Dec. 31, 2023	Dec. 31, 2023 aircraftLeasebackTransaction	Dec. 31, 2023 USD ($)	Dec. 31, 2023 aircraft	Dec. 31, 2022 USD ($)	Apr. 30, 2021	May 12, 2020
Debt Instrument [Line Items]
Long-term debt			$ 3,439.1		$ 3,642.4
Less current maturities			315.3		346.4
Less unamortized discount, net			69.0		95.8
Total			3,054.8		3,200.2
Aircraft
Debt Instrument [Line Items]
Failed aircraft sale leaseback		15		15
8.00% senior secured notes due in 2025 | 8.00% senior secured notes due in 2025
Debt Instrument [Line Items]
Stated interest rate percentage	8.00%
Long-term debt			1,110.0		$ 1,110.0
Weighted-average interest rates	8.00%				8.00%
Fixed-rate term loans due through 2039
Debt Instrument [Line Items]
Long-term debt			1,093.3		$ 1,094.7
Weighted-average interest rates	5.83%				3.52%
Unsecured term loans due through 2031 | Unsecured term loans due through 2031
Debt Instrument [Line Items]
Long-term debt			136.3		$ 136.3
Weighted-average interest rates	1.00%				1.00%
Fixed-rate class A 2015-1 EETC due through 2028
Debt Instrument [Line Items]
Long-term debt			256.6		$ 278.6
Weighted-average interest rates	4.10%				4.10%
Fixed-rate class B 2015-1 EETC due through 2024
Debt Instrument [Line Items]
Long-term debt			40.0		$ 48.0
Weighted-average interest rates	4.45%				4.45%
Fixed-rate class C 2015-1 EETC due through 2023
Debt Instrument [Line Items]
Long-term debt			0.0		$ 63.8
Weighted-average interest rates	4.93%				4.93%
Fixed-rate class AA 2017-1 EETC due through 2030
Debt Instrument [Line Items]
Long-term debt			172.2		$ 186.3
Weighted-average interest rates	3.38%				3.38%
Fixed-rate class A 2017-1 EETC due through 2030
Debt Instrument [Line Items]
Long-term debt			57.4		$ 62.1
Weighted-average interest rates	3.65%				3.65%
Fixed-rate class B 2017-1 EETC due through 2026
Debt Instrument [Line Items]
Long-term debt			48.2		$ 51.7
Weighted-average interest rates	3.80%				3.80%
Fixed-rate class C 2017-1 EETC due through 2023
Debt Instrument [Line Items]
Long-term debt			0.0		$ 85.5
Weighted-average interest rates	5.11%				5.11%
Convertible notes | Convertible notes due in 2025
Debt Instrument [Line Items]
Stated interest rate percentage	4.75%						4.75%
Long-term debt			25.1		$ 25.4
Weighted-average interest rates	4.75%				4.75%
Convertible notes | Convertible notes due in 2026
Debt Instrument [Line Items]
Stated interest rate percentage	1.00%					1.00%
Long-term debt			500.0		$ 500.0
Less unamortized discount, net			27.4
Total			$ 472.6
Weighted-average interest rates	1.00%				1.00%