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Schedule of Outstanding Debt (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Revolving line of credit & Secured long-term debt $ 287,084 $ 160,692
Less current portion (73,052) (45,163)
Total long-term debt 214,032 115,529
Revolving Line of Credit [Member]    
Debt Instrument [Line Items]    
Revolving line of credit 50,000  
Secured Equipment Notes due in 2024 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 12,388  
Secured Equipment Notes due in 2023 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 11,554  
Secured Equipment Notes due in 2022 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 32,517  
Secured Equipment Notes due in 2021 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 81,176 59,836
Secured Equipment Notes due in 2020 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 57,305 48,633
Secured Equipment Notes due in 2019 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 40,803 49,464
Secured Equipment Notes due in June 2018 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 1,280 $ 2,759
Secured Equipment Notes due in 2017 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt $ 61