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Property, Plant and Equipment
 12 Months Ended
Dec. 31, 2020
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment Property, Plant and Equipment:
Property, plant and equipment, at cost, consist of the following at December 31, 2020 and 2019 (in thousands):
Useful
Lives
(Years)		December 31,
20202019
Land—$121,330 $116,728 
Land improvements
10 – 30
115,693 83,256 
Buildings and improvements
10 – 50
354,679 337,728 
Machinery and equipment(a)
2 – 45
3,564,389 3,355,519 
Mineral rights and reserves
7 – 60
1,780,236 1,764,067 
Construction in progress—1,491,314 1,160,545 
Total$7,427,641 $6,817,843 
(a)Consists primarily of (1) short-lived production equipment components, office and building equipment and other equipment with estimated lives ranging 2 – 7 years, (2) production process equipment (intermediate components) with estimated lives ranging 8 – 19 years, (3) production process equipment (major unit components) with estimated lives ranging 20 – 29 years, and (4) production process equipment (infrastructure and other) with estimated lives ranging 30 – 45 years.
The cost of property, plant and equipment is depreciated generally by the straight-line method. Depletion of mineral rights is based on the units-of-production method. Depreciation expense, including depletion, amounted to $203.6 million,
$183.3 million and $170.0 million during the years ended December 31, 2020, 2019 and 2018, respectively. Interest capitalized on significant capital projects in 2020, 2019 and 2018 was $30.4 million, $30.2 million and $19.3 million, respectively.