Property, Plant and Equipment (Schedule of Property, Plant and Equipment) (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Property Plant And Equipment [Line Items]
Property, plant, equipment and equipment, gross	$ 2,173,566	$ 2,145,719
Less accumulated depreciation, depletion and amortization	810,194	748,356
Net property, plant and equipment	1,363,372	1,397,363
Land [Member]
Property Plant And Equipment [Line Items]
Property, plant, equipment and equipment, gross	27,197	27,853
Buildings And Improvements [Member]
Property Plant And Equipment [Line Items]
Property, plant, equipment and equipment, gross	92,604	88,079
Plant, Machinery, Mining Equipment And Transportation Vehicles [Member]
Property Plant And Equipment [Line Items]
Property, plant, equipment and equipment, gross	992,728	963,272
Mineral Rights And Reserves [Member]
Property Plant And Equipment [Line Items]
Property, plant, equipment and equipment, gross	374,326	374,326
Office And Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant, equipment and equipment, gross	9,274	8,718
Mine Development [Member]
Property Plant And Equipment [Line Items]
Property, plant, equipment and equipment, gross	557,596	566,201
Asset Retirement Obligation Asset [Member]
Property Plant And Equipment [Line Items]
Property, plant, equipment and equipment, gross	70,584	75,215
Construction In Process [Member]
Property Plant And Equipment [Line Items]
Property, plant, equipment and equipment, gross	$ 49,257	$ 42,055