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Summary of Significant Accounting Policies - Narrative (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2018
USD ($)
employee_group
Dec. 31, 2017
USD ($)
employee_group
Dec. 31, 2016
USD ($)
Accounting Policies [Abstract]      
Marketing and advertising expenses $ 6,300 $ 5,100 $ 3,200
Valuation allowance $ 254 $ 454  
Aircraft fuel expenditure concentration risk | Total operating expenses      
Concentration Risk [Line Items]      
Concentration of risk 32.00% 27.00% 24.00%
Unionized employees concentration risk | Number of employees, total      
Concentration Risk [Line Items]      
Concentration of risk 80.00% 75.00%  
Union-represented employee groups | employee_group 5 4