Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Carrying Value	$ 3,506.4	$ 2,213.6
8.00% senior secured notes due in 2025
Debt Instrument [Line Items]
Carrying Value	841.4	0.0
Fixed-rate senior term loans
Debt Instrument [Line Items]
Carrying Value	270.9	296.1
Fixed-rate term loans
Debt Instrument [Line Items]
Carrying Value	1,014.5	778.2
Unsecured term loans
Debt Instrument [Line Items]
Carrying Value		0.0
2015-1 EETC Class A
Debt Instrument [Line Items]
Carrying Value	333.6	348.6
2015-1 EETC Class B
Debt Instrument [Line Items]
Carrying Value	68.0	72.0
2015-1 EETC Class C
Debt Instrument [Line Items]
Carrying Value	92.4	98.1
2017-1 EETC Class AA
Debt Instrument [Line Items]
Carrying Value	214.4	228.4
2017-1 EETC Class A
Debt Instrument [Line Items]
Carrying Value	71.5	76.1
2017-1 EETC Class B
Debt Instrument [Line Items]
Carrying Value	60.6	70.6
2017-1 EETC Class C
Debt Instrument [Line Items]
Carrying Value	85.5	85.5
Convertible debt
Debt Instrument [Line Items]
Carrying Value	105.2	0.0
Revolving Credit Facility
Debt Instrument [Line Items]
Carrying Value	275.1
2020 Revolving Credit Facility
Debt Instrument [Line Items]
Carrying Value		160.0
Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	3,573.3	2,303.0
Estimated Fair Value | Level 3 | 8.00% senior secured notes due in 2025
Debt Instrument [Line Items]
Estimated Fair Value	853.6	0.0
Estimated Fair Value | Level 3 | Fixed-rate senior term loans
Debt Instrument [Line Items]
Estimated Fair Value	262.2	296.4
Estimated Fair Value | Level 3 | Fixed-rate term loans
Debt Instrument [Line Items]
Estimated Fair Value	1,034.7	823.6
Estimated Fair Value | Level 3 | Unsecured term loans
Debt Instrument [Line Items]
Estimated Fair Value	77.6	0.0
Estimated Fair Value | Level 3 | Revolving Credit Facility
Debt Instrument [Line Items]
Estimated Fair Value	275.1
Estimated Fair Value | Level 3 | 2020 Revolving Credit Facility
Debt Instrument [Line Items]
Estimated Fair Value		160.0
Estimated Fair Value | Level 2 | 2015-1 EETC Class A
Debt Instrument [Line Items]
Estimated Fair Value	299.4	372.2
Estimated Fair Value | Level 2 | 2015-1 EETC Class B
Debt Instrument [Line Items]
Estimated Fair Value	60.0	74.5
Estimated Fair Value | Level 2 | 2015-1 EETC Class C
Debt Instrument [Line Items]
Estimated Fair Value	76.0	100.5
Estimated Fair Value | Level 2 | 2017-1 EETC Class AA
Debt Instrument [Line Items]
Estimated Fair Value	202.6	237.0
Estimated Fair Value | Level 2 | 2017-1 EETC Class A
Debt Instrument [Line Items]
Estimated Fair Value	60.4	78.8
Estimated Fair Value | Level 2 | 2017-1 EETC Class B
Debt Instrument [Line Items]
Estimated Fair Value	48.2	72.0
Estimated Fair Value | Level 2 | 2017-1 EETC Class C
Debt Instrument [Line Items]
Estimated Fair Value	69.2	88.0
Estimated Fair Value | Level 2 | Convertible debt
Debt Instrument [Line Items]
Estimated Fair Value	$ 254.3	$ 0.0