Property, Plant and Equipment (Schedule of Property, Plant and Equipment) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 4,838,171	$ 4,676,353
Less: Accumulated Depreciation, Depletion and Amortization	2,731,643	2,554,056
Total Property, Plant and Equipment—Net	2,106,528	2,122,297
Plant and Equipment
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	2,890,970	2,757,062
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	858,153	857,031
Airshafts
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	419,100	392,266
Mine Development
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	342,405	344,139
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 327,543	$ 325,855