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Schedule of Outstanding Debt (Detail) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Secured long-term debt $ 281,625 $ 330,784
Less current portion (94,691) (101,713)
Total long-term debt 186,934 229,071
Secured Equipment Notes due in 2024 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 62,690 11,658
Secured Equipment Notes due in 2023 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 153,350 192,858
Secured Equipment Notes due in 2022 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 16,892 24,092
Secured Equipment Notes due in 2021 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 35,076 55,855
Secured Equipment Notes due in 2020 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt $ 13,617 32,904
Secured Equipment Notes due in 2019 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt   $ 13,417