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Schedule of Outstanding Debt (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Revolving line of credit & Secured long-term debt $ 266,184 $ 292,074
Less current portion (86,740) (77,266)
Total long-term debt 179,444 214,808
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 12,691 13,586
Secured Equipment Notes due in 2023 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 91,077 36,981
Secured Equipment Notes due in 2022 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 27,435 30,301
Secured Equipment Notes due in 2021 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 66,863 76,885
Secured Equipment Notes due in 2020 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 42,960 50,737
Secured Equipment Notes due in 2019 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt $ 25,158 36,178
Secured Equipment Notes due in 2018 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt   $ 2,406