Property, Plant and Equipment (Schedule of Property, Plant and Equipment) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 4,676,353	$ 4,593,395
Less Accumulated Depreciation, Depletion and Amortization	2,554,056	2,413,125
Total Property, Plant and Equipment—Net	2,122,297	2,180,270	$ 2,325,181
Plant and Equipment
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	2,757,062	2,680,453
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	857,031	861,048
Airshafts
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	392,266	381,755
Mine Development
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	344,139	344,139
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 325,855	$ 326,000