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Fair Value Accounting (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Fair Value Accounting      
Other investments $ 3,676 $ 5,462 $ 3,746
Used mill equipment (non-recurring)   5,500  
Fair Value, Inputs, Level 1 [Member]      
Fair Value Accounting      
Other investments $ 3,676 5,462  
Fair Value, Inputs, Level 3 [Member]      
Fair Value Accounting      
Used mill equipment (non-recurring)   $ 5,500