Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Carrying Value	$ 3,469.9	$ 3,580.5
8.00% senior secured notes due in 2025 | 8.00% Senior Secured Notes Due 2025
Debt Instrument [Line Items]
Carrying Value	850.0	850.0
Fixed-rate term loans
Debt Instrument [Line Items]
Carrying Value	1,272.9	1,301.9
Unsecured term loans | Payroll Support Program, CARES Act
Debt Instrument [Line Items]
Carrying Value	98.7	73.3
2015-1 EETC Class A
Debt Instrument [Line Items]
Carrying Value	322.6	322.6
2015-1 EETC Class B
Debt Instrument [Line Items]
Carrying Value	64.0	64.0
2015-1 EETC Class C
Debt Instrument [Line Items]
Carrying Value	86.6	86.6
2017-1 EETC Class AA
Debt Instrument [Line Items]
Carrying Value	207.3	214.4
2017-1 EETC Class A
Debt Instrument [Line Items]
Carrying Value	69.1	71.5
2017-1 EETC Class B
Debt Instrument [Line Items]
Carrying Value	58.2	60.6
2017-1 EETC Class C
Debt Instrument [Line Items]
Carrying Value	85.5	85.5
Convertible debt
Debt Instrument [Line Items]
Carrying Value	175.0	175.0
Revolving credit facilities
Debt Instrument [Line Items]
Carrying Value	180.0
Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	3,888.8	3,859.9
Estimated Fair Value | Revolving credit facilities
Debt Instrument [Line Items]
Carrying Value		275.1
Estimated Fair Value | Level 3 | 8.00% senior secured notes due in 2025 | 8.00% Senior Secured Notes Due 2025
Debt Instrument [Line Items]
Estimated Fair Value	881.8	886.0
Estimated Fair Value | Level 3 | Fixed-rate term loans
Debt Instrument [Line Items]
Estimated Fair Value	1,303.2	1,362.9
Estimated Fair Value | Level 3 | Unsecured term loans | Payroll Support Program, CARES Act
Debt Instrument [Line Items]
Estimated Fair Value	105.5	83.1
Estimated Fair Value | Level 3 | Revolving credit facilities
Debt Instrument [Line Items]
Estimated Fair Value	180.0	275.1
Estimated Fair Value | Level 2 | 2015-1 EETC Class A
Debt Instrument [Line Items]
Estimated Fair Value	323.4	323.4
Estimated Fair Value | Level 2 | 2015-1 EETC Class B
Debt Instrument [Line Items]
Estimated Fair Value	63.8	62.5
Estimated Fair Value | Level 2 | 2015-1 EETC Class C
Debt Instrument [Line Items]
Estimated Fair Value	81.7	77.8
Estimated Fair Value | Level 2 | 2017-1 EETC Class AA
Debt Instrument [Line Items]
Estimated Fair Value	207.9	207.4
Estimated Fair Value | Level 2 | 2017-1 EETC Class A
Debt Instrument [Line Items]
Estimated Fair Value	67.2	68.8
Estimated Fair Value | Level 2 | 2017-1 EETC Class B
Debt Instrument [Line Items]
Estimated Fair Value	56.2	56.2
Estimated Fair Value | Level 2 | 2017-1 EETC Class C
Debt Instrument [Line Items]
Estimated Fair Value	79.9	76.3
Estimated Fair Value | Level 2 | Convertible debt
Debt Instrument [Line Items]
Estimated Fair Value	$ 538.2	$ 380.3