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Schedule of Outstanding Debt (Detail) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Secured long-term debt $ 153,723 $ 160,692
Less current portion (47,582) (45,163)
Total long-term debt 106,141 115,529
Secured Equipment Notes due in June 2022 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 16,149  
Secured Equipment Notes due in 2019 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 40,374 49,464
Secured Equipment Notes due in 2021 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 53,635 59,836
Secured Equipment Notes due in 2020 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 42,333 48,633
Secured Equipment Notes due in June 2018 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt $ 1,232 $ 2,759