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Financing Arrangements - Equipment Notes (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Debt Instrument [Line Items]    
Current portion of long-term debt $ 14,599,000 $ 7,317,000
Long-term debt, excluding current maturities 650,564,000 443,259,000
Secured Debt | 2020 Equipment Note    
Debt Instrument [Line Items]    
Interest expense, debt $ 880,000 463,000
Secured Debt | 2020 Equipment Note | Minimum    
Debt Instrument [Line Items]    
Asset-based credit facility, interest rate 2.36%  
Secured Debt | 2020 Equipment Note | Maximum    
Debt Instrument [Line Items]    
Asset-based credit facility, interest rate 3.79%  
Secured Debt | 2017 Equipment Note    
Debt Instrument [Line Items]    
Long-term debt outstanding $ 31,822,000 19,715,000
Current portion of long-term debt 8,761,000 5,107,000
Long-term debt, excluding current maturities $ 23,061,000 $ 14,608,000