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Schedule of Outstanding Debt (Detail) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Secured long-term debt $ 160,692 $ 133,304
Less current portion (45,163) (32,409)
Total long-term debt 115,529 100,895
Secured Equipment Notes due in 2021 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 59,836  
Secured Equipment Notes due in 2020 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 48,633 61,206
Secured Equipment Notes due in 2019 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 49,464 67,486
Secured Equipment Notes due in June 2018 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt $ 2,759 $ 4,612