Property, Plant and Equipment (Schedule of Property, Plant and Equipment) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 5,008,180	$ 4,838,171
Less—Accumulated Depreciation, Depletion and Amortization	2,916,015	2,731,643
Total Property, Plant and Equipment—Net	2,092,165	2,106,528
Plant and Equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	3,028,514	2,890,970
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	872,909	858,153
Airshafts
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	437,003	419,100
Mine Development
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	342,706	342,405
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 327,048	$ 327,543