Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021	Aug. 31, 2020	May 12, 2020
Debt Instrument [Line Items]
Carrying Value	$ 3,138.5	$ 3,238.2
Secured Debt | 8.00% senior secured notes
Debt Instrument [Line Items]
Stated interest rate percentage	8.00%		8.00%
Carrying Value	$ 510.0	510.0
Fixed-rate term loans
Debt Instrument [Line Items]
Carrying Value	1,159.1	1,223.5
Unsecured term loans | Payroll Support Program, CARES Act
Debt Instrument [Line Items]
Carrying Value	136.3	136.3
2015-1 EETC Class A
Debt Instrument [Line Items]
Carrying Value	289.6	300.6
2015-1 EETC Class B
Debt Instrument [Line Items]
Carrying Value	52.0	56.0
2015-1 EETC Class C
Debt Instrument [Line Items]
Carrying Value	69.5	75.2
2017-1 EETC Class AA
Debt Instrument [Line Items]
Carrying Value	193.3	200.3
2017-1 EETC Class A
Debt Instrument [Line Items]
Carrying Value	64.4	66.8
2017-1 EETC Class B
Debt Instrument [Line Items]
Carrying Value	53.4	55.8
2017-1 EETC Class C
Debt Instrument [Line Items]
Carrying Value	$ 85.5	85.5
Convertible notes | 4.75% convertible notes due 2025
Debt Instrument [Line Items]
Stated interest rate percentage	4.75%			4.75%
Carrying Value	$ 25.4	28.2
Convertible notes | 1.00% convertible notes due 2026
Debt Instrument [Line Items]
Stated interest rate percentage	1.00%
Carrying Value	$ 500.0	500.0
Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	2,879.4	3,275.8
Estimated Fair Value | Level 3 | Secured Debt | 8.00% senior secured notes
Debt Instrument [Line Items]
Estimated Fair Value	468.4	530.4
Estimated Fair Value | Level 3 | Fixed-rate term loans
Debt Instrument [Line Items]
Estimated Fair Value	1,041.4	1,262.6
Estimated Fair Value | Level 3 | Unsecured term loans | Payroll Support Program, CARES Act
Debt Instrument [Line Items]
Estimated Fair Value	111.0	146.4
Estimated Fair Value | Level 2 | 2015-1 EETC Class A
Debt Instrument [Line Items]
Estimated Fair Value	271.5	311.1
Estimated Fair Value | Level 2 | 2015-1 EETC Class B
Debt Instrument [Line Items]
Estimated Fair Value	50.3	56.4
Estimated Fair Value | Level 2 | 2015-1 EETC Class C
Debt Instrument [Line Items]
Estimated Fair Value	67.7	74.0
Estimated Fair Value | Level 2 | 2017-1 EETC Class AA
Debt Instrument [Line Items]
Estimated Fair Value	175.2	203.3
Estimated Fair Value | Level 2 | 2017-1 EETC Class A
Debt Instrument [Line Items]
Estimated Fair Value	57.4	65.8
Estimated Fair Value | Level 2 | 2017-1 EETC Class B
Debt Instrument [Line Items]
Estimated Fair Value	47.5	53.6
Estimated Fair Value | Level 2 | 2017-1 EETC Class C
Debt Instrument [Line Items]
Estimated Fair Value	83.7	84.1
Estimated Fair Value | Level 2 | Convertible notes | 4.75% convertible notes due 2025
Debt Instrument [Line Items]
Estimated Fair Value	53.3	55.6
Estimated Fair Value | Level 2 | Convertible notes | 1.00% convertible notes due 2026
Debt Instrument [Line Items]
Estimated Fair Value	$ 452.0	$ 432.5