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Debt - Additional Information (Detail)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
Aircraft
Dec. 31, 2016
USD ($)
Disclosure of detailed information about borrowings [line items]    
Debt including current maturities $ 1,174,581 $ 1,184,132
Support from Export-Import Bank for net purchase price of aircraft 80.00%  
Aircraft financing maturity period 15 years  
Loan accreting percentage 100.00%  
Number of aircraft finance with SOAR structure | Aircraft 4  
Finance with SOAR structure $ 28,300 24,800
Non-cash investing and finacing transactions 89,000  
Export-import bank of united states [member]    
Disclosure of detailed information about borrowings [line items]    
Outstanding indebtedness 372,000 446,500
Borrowing on which fixed applicable interest rate is exercised 231,900 286,100
Borrowing due through 2018 [member]    
Disclosure of detailed information about borrowings [line items]    
Debt including current maturities 127,797  
Borrowing due through 2017 [member]    
Disclosure of detailed information about borrowings [line items]    
Debt including current maturities   83,500
Finance lease [member] | Fixed interest rate [member]    
Disclosure of detailed information about borrowings [line items]    
Debt including current maturities 394,200 416,300
Finance lease [member] | Floating interest rate [member]    
Disclosure of detailed information about borrowings [line items]    
Debt including current maturities $ 128,400 $ 45,400