Debt and Other Obligations - Long-term Debt (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($) aircraft	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Long-term debt, gross	$ 2,213.6	$ 2,234.5
Repayments of Long-term Debt	246.8	137.3
Long-term debt, current maturities	214.0	162.8
Less unamortized discount, net	40.4	47.7
Total	1,959.2	2,024.0
Senior loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 296.1	$ 382.4
Weighted-average interest rates	4.02%	4.10%
Junior loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 0.0	$ 31.1
Weighted-average interest rates		6.90%
Fixed-rate term loans
Debt Instrument [Line Items]
Long-term debt, gross	778.2	$ 625.1
Debt instrument, face amount	$ 152.0
Weighted-average interest rates	3.70%	3.88%
2015-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt, gross	$ 348.6	$ 378.6
Weighted-average interest rates	4.10%	4.10%
2015-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt, gross	$ 72.0	$ 80.0
Weighted-average interest rates	4.45%	4.45%
2015-1 EETC Class C
Debt Instrument [Line Items]
Long-term debt, gross	$ 98.1	$ 109.5
Weighted-average interest rates	4.93%	4.93%
2017-1 EETC Class AA
Debt Instrument [Line Items]
Long-term debt, gross	$ 228.4	$ 242.5
Weighted-average interest rates	3.38%	3.38%
2017-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt, gross	$ 76.1	$ 80.8
Weighted-average interest rates	3.65%	3.65%
2017-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt, gross	$ 70.6	$ 83.7
Weighted-average interest rates	3.80%	3.80%
2017-1 EETC Class C
Debt Instrument [Line Items]
Long-term debt, gross	$ 85.5	$ 85.5
Weighted-average interest rates	5.11%	5.11%
Revolving Credit Facility
Debt Instrument [Line Items]
Long-term debt, gross	$ 160.0	$ 135.3
Long-term debt, current maturities	50.0
Total	$ 110.0
Weighted-average interest rates	3.12%	3.72%
Minimum \| Fixed-rate term loans
Debt Instrument [Line Items]
Long-term Debt, Term	11 years
Minimum \| Fixed-rate term loans \| Fixed-rate term loans
Debt Instrument [Line Items]
Stated interest rate percentage	2.85%
Maximum \| Fixed-rate term loans
Debt Instrument [Line Items]
Long-term Debt, Term	12 years
Maximum \| Fixed-rate term loans \| Fixed-rate term loans
Debt Instrument [Line Items]
Stated interest rate percentage	3.93%
Airbus
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments \| aircraft	64
Future aircraft to be received \| aircraft	135
Airbus \| A320 \| Fixed-rate term loans
Debt Instrument [Line Items]
Number of delivered aircraft with secured debt financing commitments \| aircraft	4
Airbus \| A320NEO \| Revolving Credit Facility
Debt Instrument [Line Items]
Future aircraft to be received \| aircraft	34
Lines of Credit with Counterparties for Jet Fuel and Derivatives [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 41.5	$ 41.5
Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 160.0

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