Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	$ 1,258.7	$ 2,210.5
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	1,274.0	2,152.6
Term Loan | DIP term loan due in 2026
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	358.6
Term Loan | DIP term loan due in 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value		309.0
Term Loan | Estimated Fair Value | Level 3 | DIP term loan due in 2026
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	358.6
Term Loan | Estimated Fair Value | Level 3 | DIP term loan due in 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value		309.0
Fixed-rate term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	274.8	972.2
Fixed-rate term loans | Estimated Fair Value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	325.9	970.7
Unsecured term loans | Unsecured term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	0.0	136.3
Unsecured term loans | Estimated Fair Value | Level 3 | Unsecured term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	130.4
Enhanced Equipment Trust Certificate | 2015-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	212.6	234.6
Enhanced Equipment Trust Certificate | 2017-1 EETC Class AA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	148.3	160.3
Enhanced Equipment Trust Certificate | 2017-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	49.4	53.4
Enhanced Equipment Trust Certificate | 2017-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	0.0	44.7
Enhanced Equipment Trust Certificate | 2025 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	215.0	0.0
Enhanced Equipment Trust Certificate | Estimated Fair Value | Level 2 | 2015-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	200.4	215.8
Enhanced Equipment Trust Certificate | Estimated Fair Value | Level 2 | 2017-1 EETC Class AA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	136.8	140.4
Enhanced Equipment Trust Certificate | Estimated Fair Value | Level 2 | 2017-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	44.3	45.8
Enhanced Equipment Trust Certificate | Estimated Fair Value | Level 2 | 2017-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	40.5
Enhanced Equipment Trust Certificate | Estimated Fair Value | Level 2 | 2025 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	208.0	0.0
Revolving credit facility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	0.0	300.0
Revolving credit facility | Estimated Fair Value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 0.0	300.0
Secured Debt | DIP term loan due in 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value		$ 309.0