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Property, Plant and Equipment (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Property, Plant and Equipment [Abstract]    
Land $ 123,518 $ 118,428
Land improvements 63,349 63,328
Buildings and improvements 251,980 245,482
Machinery and equipment 2,780,478 2,627,667
Long-term mineral rights and production equipment costs 696,033 675,832
Construction in progress 883,705 393,598
Total $ 4,799,063 $ 4,124,335