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Pension Plans (Details) - Pension Plan - Consenting agreement entered with local bargaining union - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 12, 2012
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Plans            
Insolvent period   20 years        
Estimated liability upon withdrawal from plan       $ 99,300 $ 99,800 $ 81,600
Percentage of annual compounded surcharge for rehabilitation   5.00%        
Percentage of interim surcharge 5.00%          
Pension expense   $ 1,557 $ 1,504 2,866 2,961  
Surcharges   $ 549 $ 530 $ 1,010 $ 948