Debt and Other Obligations - Schedule of Long-term Debt (Details) $ in Millions	Sep. 30, 2025 USD ($) aircraft	Dec. 31, 2024 USD ($)
Debt Instrument [Line Items]
Long-term debt	$ 931.0	$ 2,210.5
Less current maturities, net	119.2	436.3
Less unamortized discounts, net	61.6	13.2
Total	$ 750.2	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	$ 294.6	$ 972.2
Weighted-average interest rate	5.27%	6.44%
Unsecured term loans due in 2031
Debt Instrument [Line Items]
Long-term debt	$ 0.0	$ 136.3
Unsecured term loans due in 2031 | Payroll Support Program, CARES Act
Debt Instrument [Line Items]
Long-term debt		$ 136.3
Weighted-average interest rate	2.50%	1.00%
Enhanced Equipment Trust Certificate | 2015-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt	$ 223.6	$ 234.6
Weighted-average interest rate	4.03%	4.10%
Enhanced Equipment Trust Certificate | 2017-1 EETC Class AA
Debt Instrument [Line Items]
Long-term debt	$ 152.3	$ 160.3
Weighted-average interest rate	3.40%	3.38%
Enhanced Equipment Trust Certificate | 2017-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt	$ 45.5	$ 53.4
Weighted-average interest rate	3.53%	3.65%
Enhanced Equipment Trust Certificate | 2017-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt	$ 0.0	$ 44.7
Weighted-average interest rate		3.80%
Enhanced Equipment Trust Certificate | 2025-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt	$ 215.0	$ 0.0
Weighted-average interest rate	11.00%
Exit secured notes | 2030 Senior Notes
Debt Instrument [Line Items]
Long-term debt	$ 0.0	0.0
Weighted-average interest rate	12.00%
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%