Note 10 - Property, Plant and Equipment - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Total Property, Plant and Equipment	$ 5,250,805	$ 5,143,696
Less: Accumulated Depreciation, Depletion and Amortization	3,272,255	3,094,634
Total Property, Plant and Equipment - Net	1,978,550	2,049,062
Coal and Other Plant and Equipment [Member]
Total Property, Plant and Equipment	3,213,512	3,134,149
Mining Properties and Mineral Rights [Member]
Total Property, Plant and Equipment	877,271	874,567
Airshafts [Member]
Total Property, Plant and Equipment	473,866	452,976
Mine Development [Member]
Total Property, Plant and Equipment	357,479	354,691
Coal Advance Mining Royalties [Member]
Total Property, Plant and Equipment	$ 328,677	$ 327,313