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Schedule of Outstanding Debt (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Revolving line of credit & Secured long-term debt $ 274,557 $ 292,074
Less current portion (80,004) (77,266)
Total long-term debt 194,553 214,808
Revolving Line of Credit [Member]    
Debt Instrument [Line Items]    
Revolving line of credit 30,000 45,000
Secured Equipment Notes due in 2024 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 13,119 13,586
Secured Equipment Notes due in 2023 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 52,186 36,981
Secured Equipment Notes due in 2022 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 28,709 30,301
Secured Equipment Notes due in 2021 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 70,999 76,885
Secured Equipment Notes due in 2020 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 46,777 50,737
Secured Equipment Notes due in 2019 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 30,748 36,178
Secured Equipment Notes due in 2018 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt $ 2,019 $ 2,406