Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Apr. 30, 2021	May 12, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	$ 3,439.1	$ 3,642.4
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 3,253.5	3,355.0
Secured Debt | 8.00% senior secured notes due in 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate percentage	8.00%
Carrying Value	$ 1,110.0	1,110.0
Secured Debt | Estimated Fair Value | Level 3 | 8.00% senior secured notes due in 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	1,121.9	1,085.0
Fixed-rate term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	1,093.3	1,094.7
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	1,099.9	1,003.9
Unsecured term loans due through 2031 | Unsecured term loans due through 2031
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	136.3	136.3
Unsecured term loans due through 2031 | Estimated Fair Value | Level 3 | Unsecured term loans due through 2031
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	128.3	116.0
2015-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	256.6	278.6
2015-1 EETC Class A | Estimated Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	230.8	247.5
2015-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	40.0	48.0
2015-1 EETC Class B | Estimated Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	39.4	45.6
2015-1 EETC Class C
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	0.0	63.8
2015-1 EETC Class C | Estimated Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	63.1
2017-1 EETC Class AA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	172.2	186.3
2017-1 EETC Class AA | Estimated Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	149.6	161.6
2017-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	57.4	62.1
2017-1 EETC Class A | Estimated Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	48.5	52.3
2017-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	48.2	51.7
2017-1 EETC Class B | Estimated Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	42.9	44.9
2017-1 EETC Class C
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	0.0	85.5
2017-1 EETC Class C | Estimated Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 0.0	85.1
Convertible notes | Convertible notes due in 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate percentage	4.75%			4.75%
Carrying Value	$ 25.1	25.4
Convertible notes | Convertible notes due in 2026
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate percentage	1.00%		1.00%
Carrying Value	$ 500.0	500.0
Convertible notes | Estimated Fair Value | Level 2 | Convertible notes due in 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	42.3	44.9
Convertible notes | Estimated Fair Value | Level 2 | Convertible notes due in 2026
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 349.9	$ 405.1