Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	$ 1,539.6	$ 1,012.4
Senior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	417.9	451.9
Junior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	39.3	47.1
Fixed-rate loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	518.0	0.0
2015-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	408.6	409.8
2015-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	92.0	103.6
2017-1 EETC Class AA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	37.5	0.0
2017-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	12.5	0.0
2017-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	13.8	0.0
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	1,583.2	1,033.7
Estimate of Fair Value Measurement [Member] | Level 3 | Senior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	435.3	463.9
Estimate of Fair Value Measurement [Member] | Level 3 | Junior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	40.4	48.1
Estimate of Fair Value Measurement [Member] | Level 3 | Fixed-rate loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	528.6
Estimate of Fair Value Measurement [Member] | Level 2 | 2015-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	420.9	416.0
Estimate of Fair Value Measurement [Member] | Level 2 | 2015-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	94.2	$ 105.7
Estimate of Fair Value Measurement [Member] | Level 2 | 2017-1 EETC Class AA
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	37.4
Estimate of Fair Value Measurement [Member] | Level 2 | 2017-1 EETC Class A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	12.6
Estimate of Fair Value Measurement [Member] | Level 2 | 2017-1 EETC Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 13.8