Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) employee_group	Dec. 31, 2016 USD ($) employee_group	Dec. 31, 2015 USD ($)
Concentration Risk [Line Items]
Marketing and advertising expenses	$ 5,100	$ 3,200	$ 3,500
Valuation allowance	$ 454	$ 0
Aircraft fuel expenditure concentration risk \| Total operating expenses
Concentration Risk [Line Items]
Concentration of risk	27.00%	24.00%	28.00%
Unionized employees concentration risk \| Number of employees, total
Concentration Risk [Line Items]
Concentration of risk	75.00%	73.00%
Union-represented employee groups \| employee_group	4	4

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- Definition

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.

+ References

No definition available.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
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- Definition

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

The total expense recognized in the period for promotion, public relations, and brand or product advertising.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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