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Schedule of Outstanding Debt (Detail) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Secured long-term debt $ 148,910 $ 160,692
Less current portion (44,750) (45,163)
Total long-term debt 104,160 115,529
Secured Equipment Notes due in 2021 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 56,599 59,836
Secured Equipment Notes due in 2020 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 45,492 48,633
Secured Equipment Notes due in 2019 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 44,928 49,464
Secured Equipment Notes due in June 2018 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt $ 1,891 $ 2,759