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Schedule of Outstanding Debt (Detail) - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Debt Instrument [Line Items]    
Interim Funding for a deposit on equipment expected to be received in the fourth quarter of 2020; interest paid monthly at a variable rate $ 3,820  
Total debt 235,968 $ 281,625
Less current portion (86,964) (94,691)
Total long-term debt 149,004 186,934
Secured Equipment Notes due in 2025 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 22,036  
Secured Equipment Notes due in 2024 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 53,497 62,690
Secured Equipment Notes due in 2023 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 123,314 153,350
Secured Equipment Notes due in 2022 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 10,687 16,892
Secured Equipment Notes due in 2021 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt 20,043 35,076
Secured Equipment Notes due in 2020 [Member]    
Debt Instrument [Line Items]    
Secured long-term debt $ 2,571 $ 13,617