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Property, Plant, and Equipment, at Cost
 12 Months Ended
Dec. 31, 2020
Property, Plant and Equipment [Abstract]  
Property, Plant, and Equipment, at Cost Property, Plant, and Equipment, at Cost
 December 31,
(in thousands)
20202019
Land$37,796 $40,938 
Land improvements58,646 57,635 
Leasehold improvements2,146 2,122 
Buildings183,234 174,600 
Machinery and equipment1,188,396 1,161,409 
Construction in progress73,234 29,228 
1,543,452 1,465,932 
Less accumulated depreciation and amortization878,305 830,493 
Net property, plant, and equipment$665,147 $635,439 
We depreciate the cost of property, plant, and equipment by the straight-line method over the following estimated useful lives:
Land improvements
9 - 40 years
Buildings
10 - 46 years
Machinery and equipment
3 - 30 years
Depreciation expense was $61 million in 2020, $64 million in 2019, and $63 million in 2018.