Note 11 - Property, Plant and Equipment - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Total Property, Plant and Equipment	$ 5,131,739	$ 5,143,696
Less: Accumulated Depreciation, Depletion and Amortization	3,117,651	3,094,634
Total Property, Plant and Equipment, Net	2,014,088	2,049,062
Coal and Other Plant and Equipment [Member]
Total Property, Plant and Equipment	3,118,268	3,134,149
Mining Properties and Mineral Rights [Member]
Total Property, Plant and Equipment	875,079	874,567
Airshafts [Member]
Total Property, Plant and Equipment	456,083	452,976
Mine Development [Member]
Total Property, Plant and Equipment	356,136	354,691
Coal Advance Mining Royalties [Member]
Total Property, Plant and Equipment	$ 326,173	$ 327,313