Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Apr. 30, 2021	May 12, 2020
Debt Instrument [Line Items]
Carrying Value	$ 3,287.2	$ 3,439.1
Secured Debt | 8.00% senior secured notes
Debt Instrument [Line Items]
Stated interest rate percentage	8.00%	8.00%	8.00%
Carrying Value	$ 1,110.0	$ 1,110.0
Fixed-rate term loans
Debt Instrument [Line Items]
Carrying Value	1,011.8	1,093.3
Unsecured term loans | Unsecured term loans
Debt Instrument [Line Items]
Carrying Value	136.3	136.3
2015-1 EETC Class A
Debt Instrument [Line Items]
Carrying Value	245.6	256.6
2015-1 EETC Class B
Debt Instrument [Line Items]
Carrying Value	0.0	40.0
2017-1 EETC Class AA
Debt Instrument [Line Items]
Carrying Value	160.3	172.2
2017-1 EETC Class A
Debt Instrument [Line Items]
Carrying Value	53.4	57.4
2017-1 EETC Class B
Debt Instrument [Line Items]
Carrying Value	$ 44.7	$ 48.2
Convertible Notes | 4.75% convertible notes due 2025
Debt Instrument [Line Items]
Stated interest rate percentage	4.75%	4.75%			4.75%
Carrying Value	$ 25.1	$ 25.1
Convertible Notes | 1.00% convertible notes due 2026
Debt Instrument [Line Items]
Stated interest rate percentage	1.00%	1.00%		1.00%
Carrying Value	$ 500.0	$ 500.0
Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	2,932.1	3,253.5
Estimated Fair Value | Level 3 | Secured Debt | 8.00% senior secured notes
Debt Instrument [Line Items]
Estimated Fair Value	1,125.0	1,121.9
Estimated Fair Value | Level 3 | Fixed-rate term loans
Debt Instrument [Line Items]
Estimated Fair Value	1,049.6	1,099.9
Estimated Fair Value | Level 3 | Unsecured term loans | Unsecured term loans
Debt Instrument [Line Items]
Estimated Fair Value	136.9	128.3
Estimated Fair Value | Level 2 | 2015-1 EETC Class A
Debt Instrument [Line Items]
Estimated Fair Value	228.4	230.8
Estimated Fair Value | Level 2 | 2015-1 EETC Class B
Debt Instrument [Line Items]
Estimated Fair Value	0.0	39.4
Estimated Fair Value | Level 2 | 2017-1 EETC Class AA
Debt Instrument [Line Items]
Estimated Fair Value	139.4	149.6
Estimated Fair Value | Level 2 | 2017-1 EETC Class A
Debt Instrument [Line Items]
Estimated Fair Value	44.3	48.5
Estimated Fair Value | Level 2 | 2017-1 EETC Class B
Debt Instrument [Line Items]
Estimated Fair Value	39.1	42.9
Estimated Fair Value | Level 2 | Convertible Notes | 4.75% convertible notes due 2025
Debt Instrument [Line Items]
Estimated Fair Value	16.4	42.3
Estimated Fair Value | Level 2 | Convertible Notes | 1.00% convertible notes due 2026
Debt Instrument [Line Items]
Estimated Fair Value	$ 153.0	$ 349.9