Debt and Other Obligations - Schedule of Long-term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 3,506.4	$ 2,213.6
Current maturities of long-term debt	372.8	214.0
Less unamortized discounts, net	60.0	40.4
Long-term debt and capital leases, less current maturities	3,073.6	1,959.2
8.00% senior secured notes due in 2025
Debt Instrument [Line Items]
Long-term debt	$ 841.4	0.0
Weighted-average interest rate	8.00%
Fixed-rate senior term loans
Debt Instrument [Line Items]
Long-term debt	$ 270.9	$ 296.1
Weighted-average interest rate	3.55%	4.02%
Fixed-rate term loans
Debt Instrument [Line Items]
Long-term debt	$ 1,014.5	$ 778.2
Weighted-average interest rate	3.34%	3.70%
Unsecured term loans
Debt Instrument [Line Items]
Long-term debt		$ 0.0
Weighted-average interest rate	1.00%
2015-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt	$ 333.6	$ 348.6
Weighted-average interest rate	4.10%	4.10%
2015-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt	$ 68.0	$ 72.0
Weighted-average interest rate	4.45%	4.45%
2015-1 EETC Class C
Debt Instrument [Line Items]
Long-term debt	$ 92.4	$ 98.1
Weighted-average interest rate	4.93%	4.93%
2017-1 EETC Class AA
Debt Instrument [Line Items]
Long-term debt	$ 214.4	$ 228.4
Weighted-average interest rate	3.38%	3.38%
2017-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt	$ 71.5	$ 76.1
Weighted-average interest rate	3.65%	3.65%
2017-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt	$ 60.6	$ 70.6
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	$ 105.2	$ 0.0
Weighted-average interest rate	4.75%
Revolving credit facility due in 2021
Debt Instrument [Line Items]
Long-term debt	$ 95.1
Weighted-average interest rate	1.56%
2020 Revolving Credit Facility
Debt Instrument [Line Items]
Long-term debt		$ 160.0
Weighted-average interest rate		3.12%
Revolving credit facility due in 2022
Debt Instrument [Line Items]
Long-term debt	$ 180.0	$ 0.0
Weighted-average interest rate	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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