Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Carrying Value	$ 2,340.0	$ 2,213.6
Fixed-rate senior term loans
Debt Instrument [Line Items]
Carrying Value	287.8	296.1
Fixed-rate term loans
Debt Instrument [Line Items]
Carrying Value	927.4	778.2
2015-1 EETC Class A
Debt Instrument [Line Items]
Carrying Value	348.6	348.6
2015-1 EETC Class B
Debt Instrument [Line Items]
Carrying Value	72.0	72.0
2015-1 EETC Class C
Debt Instrument [Line Items]
Carrying Value	98.1	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	85.3	85.5
Estimated Fair Value
Debt Instrument [Line Items]
Estimated Fair Value	2,197.1	2,303.0
Estimated Fair Value | Level 3 | Fixed-rate senior term loans
Debt Instrument [Line Items]
Estimated Fair Value	269.4	296.4
Estimated Fair Value | Level 3 | Fixed-rate term loans
Debt Instrument [Line Items]
Estimated Fair Value	916.9	823.6
Estimated Fair Value | Level 3 | Revolving credit facilities
Debt Instrument [Line Items]
Estimated Fair Value	160.0	160.0
Estimated Fair Value | Level 2 | 2015-1 EETC Class A
Debt Instrument [Line Items]
Estimated Fair Value	313.8	372.2
Estimated Fair Value | Level 2 | 2015-1 EETC Class B
Debt Instrument [Line Items]
Estimated Fair Value	62.6	74.5
Estimated Fair Value | Level 2 | 2015-1 EETC Class C
Debt Instrument [Line Items]
Estimated Fair Value	83.6	100.5
Estimated Fair Value | Level 2 | 2017-1 EETC Class AA
Debt Instrument [Line Items]
Estimated Fair Value	201.5	237.0
Estimated Fair Value | Level 2 | 2017-1 EETC Class A
Debt Instrument [Line Items]
Estimated Fair Value	64.6	78.8
Estimated Fair Value | Level 2 | 2017-1 EETC Class B
Debt Instrument [Line Items]
Estimated Fair Value	52.5	72.0
Estimated Fair Value | Level 2 | 2017-1 EETC Class C
Debt Instrument [Line Items]
Estimated Fair Value	$ 72.2	$ 88.0