Debt and Other Obligations - Long-term Debt (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,539,600	$ 1,012,400
Less current maturities	115,430	84,354
Less unamortized discount, net	36,700	30,600
Total	1,387,498	897,359
Senior loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 417,900	$ 451,900
Weighted-average interest rates	4.10%	4.10%
Junior loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 39,300	$ 47,100
Weighted-average interest rates	6.90%	6.90%
Fixed-rate loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 518,000	$ 0
Weighted-average interest rates	3.83%
2015-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt, gross	$ 408,600	$ 409,800
Weighted-average interest rates	4.10%	4.10%
2015-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt, gross	$ 92,000	$ 103,600
Weighted-average interest rates	4.45%	4.45%
2017-1 EETC Class AA
Debt Instrument [Line Items]
Long-term debt, gross	$ 37,500	$ 0
Weighted-average interest rates	3.38%
2017-1 EETC Class A
Debt Instrument [Line Items]
Long-term debt, gross	$ 12,500	0
Weighted-average interest rates	3.65%
2017-1 EETC Class B
Debt Instrument [Line Items]
Long-term debt, gross	$ 13,800	$ 0
Weighted-average interest rates	3.80%
Minimum \| Fixed-rate loans
Debt Instrument [Line Items]
Debt instrument, term	10 years
Maximum \| Fixed-rate loans
Debt Instrument [Line Items]
Debt instrument, term	12 years

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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

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- Definition

Amount, after accumulated amortization, of debt discount.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

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- Definition

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.

+ References

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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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Weighted average interest rate of long-term debt outstanding calculated at point in time.

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