Debt and Other Obligations - Schedule of Long-term Debt (Details) $ in Millions	Mar. 31, 2025 USD ($) aircraft	Dec. 31, 2024 USD ($)
Debt Instrument [Line Items]
Long-term debt	$ 2,581.3	$ 2,210.5
Less current maturities, net (2)	191.1	436.3
Less unamortized discounts, net	158.9	13.2
Total	$ 2,231.3	1,761.0
Aircraft
Debt Instrument [Line Items]
Failed aircraft sale leaseback | aircraft	18
Term Loan | DIP Facility
Debt Instrument [Line Items]
Long-term debt	$ 0.0	$ 309.0
Weighted-average interest rate		11.82%
Fixed-rate loans due through 2039
Debt Instrument [Line Items]
Long-term debt	$ 904.9	$ 972.2
Weighted-average interest rate	6.44%	6.44%
Unsecured term loans due in 2031 | Payroll Support Program, CARES Act
Debt Instrument [Line Items]
Long-term debt	$ 136.3	$ 136.3
Weighted-average interest rate	1.00%	1.00%
Enhanced Equipment Trust Certificate | 2015-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt	$ 234.6	$ 234.6
Weighted-average interest rate	4.10%	4.10%
Enhanced Equipment Trust Certificate | 2017-1 EETC Class AA
Debt Instrument [Line Items]
Long-term debt	$ 154.3	$ 160.3
Weighted-average interest rate	3.38%	3.38%
Enhanced Equipment Trust Certificate | 2017-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt	$ 51.4	$ 53.4
Weighted-average interest rate	3.65%	3.65%
Enhanced Equipment Trust Certificate | 2017-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt	$ 43.0	$ 44.7
Weighted-average interest rate	3.80%	3.80%
Enhanced Equipment Trust Certificate | Equipment Notes, Series B(R) 2025
Debt Instrument [Line Items]
Long-term debt		$ 0.0
Weighted-average interest rate	11.00%
Exit Secured Notes | Senior Notes
Debt Instrument [Line Items]
Long-term debt	$ 841.8	0.0
Weighted-average interest rate	12.00%
Revolving credit facility | Revolving credit facility due in 2026
Debt Instrument [Line Items]
Long-term debt	$ 0.0	300.0
Revolving credit facility | Revolving credit facility due in 2028
Debt Instrument [Line Items]
Long-term debt	$ 0.0	$ 300.0
Weighted-average interest rate		6.67%