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Financing Arrangements - Equipment Notes (Details)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2021
USD ($)
Jun. 30, 2020
USD ($)
Jun. 30, 2021
USD ($)
Jun. 30, 2020
USD ($)
Dec. 31, 2020
USD ($)
Aug. 31, 2020
financing_agreement
Debt Instrument [Line Items]            
Number of long-term financing agreements | financing_agreement           4
Long-term debt $ 1,081,969   $ 1,081,969   $ 665,289  
Long-term debt, excluding current maturities 1,064,429   1,064,429   $ 650,564  
Interest expense, debt $ 221 $ 212 $ 460 $ 377