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Expendable Parts and Supplies - Disclosure of Expendable Parts and Supplies (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Classes of current inventories [abstract]      
Material for repair and maintenance $ 79,424 $ 71,876  
Other inventories 3,058 3,101  
Expendable parts and supplies, gross 82,482 74,977  
Allowance for obsolescence (657) (475)  
Expendable parts and supplies $ 81,825 $ 74,502 $ 62,247