Fair Value Measurements - Long-term Debt (Details) $ in Millions	Sep. 30, 2025 USD ($) aircraft	Dec. 31, 2024 USD ($)
Debt Instrument [Line Items]
Carrying Value	$ 931.0	$ 2,210.5
Aircraft
Debt Instrument [Line Items]
Failed aircraft sale leaseback | aircraft	18
Fixed-rate term loans
Debt Instrument [Line Items]
Carrying Value	$ 294.6	972.2
Fixed-rate term loans | Level 3 | Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	326.5	970.7
Unsecured term loans
Debt Instrument [Line Items]
Carrying Value	0.0	136.3
Unsecured term loans | Level 3 | Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	0.0	130.4
Long Term Debt Excluding Liabilities Subject To Compromise
Debt Instrument [Line Items]
Carrying Value	931.0	2,210.5
Long Term Debt Excluding Liabilities Subject To Compromise | Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	919.6	2,152.6
DIP Facility | Term Loan
Debt Instrument [Line Items]
Carrying Value	0.0	309.0
DIP Facility | Term Loan | Level 3 | Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	0.0	309.0
2015-1 EETC Class A | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Carrying Value	223.6	234.6
2015-1 EETC Class A | Enhanced Equipment Trust Certificate | Level 2 | Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	209.1	215.8
2017-1 EETC Class AA | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Carrying Value	152.3	160.3
2017-1 EETC Class AA | Enhanced Equipment Trust Certificate | Level 2 | Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	139.4	140.4
2017-1 EETC Class A | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Carrying Value	45.5	53.4
2017-1 EETC Class A | Enhanced Equipment Trust Certificate | Level 2 | Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	40.3	45.8
2017-1 EETC Class B | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Carrying Value	0.0	44.7
2017-1 EETC Class B | Enhanced Equipment Trust Certificate | Level 2 | Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	0.0	40.5
2025-1 EETC Class B | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Carrying Value	215.0	0.0
2025-1 EETC Class B | Enhanced Equipment Trust Certificate | Level 2 | Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	204.3	0.0
Revolving credit facility due in 2026 | Revolving credit facility
Debt Instrument [Line Items]
Carrying Value	0.0	300.0
Revolving credit facility due in 2026 | Revolving credit facility | Level 3 | Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	$ 0.0	$ 300.0