Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Carrying Value	$ 2,664.8	$ 2,213.6
Fixed-rate senior term loans
Debt Instrument [Line Items]
Carrying Value	279.4	296.1
Fixed-rate term loans
Debt Instrument [Line Items]
Carrying Value	992.2	778.2
Unsecured term loans
Debt Instrument [Line Items]
Carrying Value	60.4	0.0
2015-1 EETC Class A
Debt Instrument [Line Items]
Carrying Value	333.5	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	221.4	228.4
2017-1 EETC Class A
Debt Instrument [Line Items]
Carrying Value	73.8	76.1
2017-1 EETC Class B
Debt Instrument [Line Items]
Carrying Value	65.6	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	101.4	0.0
Revolving Credit Facility
Debt Instrument [Line Items]
Carrying Value	291.2
2020 Revolving Credit Facility
Debt Instrument [Line Items]
Carrying Value		160.0
Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	2,684.5	2,303.0
Estimated Fair Value | Level 3 | Fixed-rate senior term loans
Debt Instrument [Line Items]
Estimated Fair Value	262.8	296.4
Estimated Fair Value | Level 3 | Fixed-rate term loans
Debt Instrument [Line Items]
Estimated Fair Value	976.8	823.6
Estimated Fair Value | Level 3 | Unsecured term loans
Debt Instrument [Line Items]
Estimated Fair Value	60.4	0.0
Estimated Fair Value | Level 3 | Revolving Credit Facility
Debt Instrument [Line Items]
Estimated Fair Value	291.2
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	321.9	372.2
Estimated Fair Value | Level 2 | 2015-1 EETC Class B
Debt Instrument [Line Items]
Estimated Fair Value	59.3	74.5
Estimated Fair Value | Level 2 | 2015-1 EETC Class C
Debt Instrument [Line Items]
Estimated Fair Value	65.8	100.5
Estimated Fair Value | Level 2 | 2017-1 EETC Class AA
Debt Instrument [Line Items]
Estimated Fair Value	208.1	237.0
Estimated Fair Value | Level 2 | 2017-1 EETC Class A
Debt Instrument [Line Items]
Estimated Fair Value	61.8	78.8
Estimated Fair Value | Level 2 | 2017-1 EETC Class B
Debt Instrument [Line Items]
Estimated Fair Value	51.8	72.0
Estimated Fair Value | Level 2 | 2017-1 EETC Class C
Debt Instrument [Line Items]
Estimated Fair Value	60.5	88.0
Estimated Fair Value | Level 2 | Convertible debt
Debt Instrument [Line Items]
Estimated Fair Value	$ 264.1	$ 0.0