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Property, Plant, Equipment – Net
 12 Months Ended
Dec. 31, 2020
Property, Plant and Equipment [Abstract]  
Property, Plant, Equipment – Net Property, Plant, Equipment – Net
 December 31,
 20202019
Land and improvements
$6,396 $6,396 
Machinery and equipment
1,430,192 1,337,234 
Buildings and improvements
201,728 184,951 
Construction in progress
47,000 97,143 
 1,685,316 1,625,724 
Less – accumulated depreciation
(919,847)(869,843)
Total property, plant, equipment – net
$765,469 $755,881 

Capitalized interest was $5,580, $6,359 and $3,619 for the years ended December 31, 2020, 2019 and 2018, respectively.

Depreciation expense was $57,240, $53,424 and $49,729 for the years ended December 31, 2020, 2019 and 2018, respectively.