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Schedule of Outstanding Debt (Detail) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Secured long-term debt $ 315,460 $ 330,784
Less current portion (100,227) (101,713)
Total long-term debt 215,233 229,071
Secured Equipment Notes due in 2024 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 21,660 11,658
Secured Equipment Notes due in 2023 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 183,124 192,858
Secured Equipment Notes due in 2022 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 23,179 24,092
Secured Equipment Notes due in 2021 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 50,824 55,855
Secured Equipment Notes due in 2020 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 27,598 32,904
Secured Equipment Notes due in 2019 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt $ 9,075 $ 13,417