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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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Face (par) amount of debt instrument at time of issuance.

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Amount, after accumulated amortization, of debt discount.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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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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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Period between issuance and maturity of long-term debt, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Debt and Other Obligations (Details) $ in Millions	3 Months Ended		6 Months Ended
Debt and Other Obligations (Details) $ in Millions	Jun. 30, 2019 USD ($) aircraft	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) aircraft	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Long-term debt	$ 2,243.5		$ 2,243.5		$ 2,234.5
Less current maturities	175.5		175.5		162.8
Less unamortized discounts, net	(44.2)		(44.2)		(47.7)
Total	2,023.8		2,023.8		2,024.0
Repayments of debt obligations	47.9	$ 42.0	85.8	$ 60.6
Senior Loans [Member]
Debt Instrument [Line Items]
Long-term debt	$ 364.0		$ 364.0		382.4
Weighted-average interest rate	4.10%	4.10%	4.10%	4.10%
Junior Loans [Member]
Debt Instrument [Line Items]
Long-term debt	$ 26.8		$ 26.8		31.1
Weighted-average interest rate	6.90%	6.90%	6.90%	6.90%
Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Principal amount	$ 70.0		$ 70.0
Long-term debt	$ 672.7		$ 672.7		625.1
Weighted-average interest rate	3.48%	3.83%	3.48%	3.83%
Equipment Notes, Series A 2015-1 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 363.6		$ 363.6		378.6
Weighted-average interest rate	4.10%	4.10%	4.10%	4.10%
Equipment Notes, Series B 2015-1 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 76.0		$ 76.0		80.0
Weighted-average interest rate	4.45%	4.45%	4.45%	4.45%
Equipment Notes, Series C 2015-1 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 103.8		$ 103.8		109.5
Weighted-average interest rate	4.93%	4.93%	4.93%	4.93%
Equipment Notes, Series AA 2017-1 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 235.4		$ 235.4		242.5
Weighted-average interest rate	3.38%	3.38%	3.38%	3.38%
Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 78.5		$ 78.5		80.8
Weighted-average interest rate	3.65%	3.65%	3.65%	3.65%
Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 77.2		$ 77.2		83.7
Weighted-average interest rate	3.80%	3.80%	3.80%	3.80%
Equipment Notes, Series C 2017-1 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 85.5		$ 85.5		85.5
Weighted-average interest rate	5.11%	5.11%	5.11%	5.11%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Long-term debt	$ 160.0		$ 160.0		$ 135.3
Weighted-average interest rate	3.86%		3.87%
Fixed Rate Loan [Member] \| Maximum
Debt Instrument [Line Items]
Long-term debt, term	12 years		12 years
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 160.0		$ 160.0
Airbus [Member] \| A-320-Neo [Member]
Debt Instrument [Line Items]
Future aircraft to be received \| aircraft	43		43
Airbus [Member] \| A-320-Neo [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Future aircraft to be received \| aircraft	43		43