Debt and Other Obligations - Schedule of Long-term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Long-term debt	$ 3,469.9	$ 3,580.5
Current maturities of long-term debt	356.0	383.5
Less unamortized discounts, net	66.5	131.4
Long-term debt and capital leases, less current maturities	3,047.4	3,065.6
8.00% senior secured notes due in 2025 \| 8.00% Senior Secured Notes Due 2025
Debt Instrument [Line Items]
Long-term debt	$ 850.0	$ 850.0
Weighted-average interest rate	8.00%	8.00%
Fixed-rate term loans
Debt Instrument [Line Items]
Long-term debt	$ 1,272.9	$ 1,301.9
Weighted-average interest rate	3.36%	3.36%
Unsecured term loans \| Payroll Support Program, CARES Act
Debt Instrument [Line Items]
Long-term debt	$ 98.7	$ 73.3
Weighted-average interest rate	1.00%	1.00%
2015-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt	$ 322.6	$ 322.6
Weighted-average interest rate	4.10%	4.10%
2015-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt	$ 64.0	$ 64.0
Weighted-average interest rate	4.45%	4.45%
2015-1 EETC Class C
Debt Instrument [Line Items]
Long-term debt	$ 86.6	$ 86.6
Weighted-average interest rate	4.93%	4.93%
2017-1 EETC Class AA
Debt Instrument [Line Items]
Long-term debt	$ 207.3	$ 214.4
Weighted-average interest rate	3.38%	3.38%
2017-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt	$ 69.1	$ 71.5
Weighted-average interest rate	3.65%	3.65%
2017-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt	$ 58.2	$ 60.6
Weighted-average interest rate	3.80%	3.80%
2017-1 EETC Class C
Debt Instrument [Line Items]
Long-term debt	$ 85.5	$ 85.5
Weighted-average interest rate	5.11%	5.11%
Convertible debt
Debt Instrument [Line Items]
Long-term debt	$ 175.0	$ 175.0
Weighted-average interest rate	4.75%	4.75%
Revolving credit facility due in 2021
Debt Instrument [Line Items]
Long-term debt	$ 0.0	$ 95.1
Weighted-average interest rate		1.55%
Revolving credit facility due in 2024
Debt Instrument [Line Items]
Long-term debt	$ 180.0	$ 180.0
Weighted-average interest rate	2.11%	2.15%

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Amount, after accumulated amortization, of debt discount.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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