Note 10 - Property, Plant and Equipment - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Total Property, Plant and Equipment	$ 5,143,696	$ 5,008,180
Less: Accumulated Depreciation, Depletion and Amortization	3,094,634	2,916,015
Total Property, Plant and Equipment - Net	2,049,062	2,092,165
Coal and Other Plant and Equipment [Member]
Total Property, Plant and Equipment	3,134,149	3,028,514
Mining Properties and Mineral Rights [Member]
Total Property, Plant and Equipment	874,567	872,909
Airshafts [Member]
Total Property, Plant and Equipment	452,976	437,003
Mine Development [Member]
Total Property, Plant and Equipment	354,691	342,706
Coal Advance Mining Royalties [Member]
Total Property, Plant and Equipment	$ 327,313	$ 327,048