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Schedule of Outstanding Debt (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Revolving line of credit & Secured long-term debt $ 292,074 $ 160,692
Less current portion (77,266) (45,163)
Total long-term debt 214,808 115,529
Revolving Line of Credit [Member]    
Debt Instrument [Line Items]    
Revolving line of credit 45,000  
Secured Equipment Notes due in 2024 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 13,586  
Secured Equipment Notes due in 2023 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 36,981  
Secured Equipment Notes due in 2022 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 30,301  
Secured Equipment Notes due in 2021 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 76,885 59,836
Secured Equipment Notes due in 2020 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 50,737 48,633
Secured Equipment Notes due in 2019 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 36,178 49,464
Equipment Notes Due June Two Thousand Eighteen    
Debt Instrument [Line Items]    
Secured long-term debt $ 2,406 $ 2,759