Note 10 - Property, Plant and Equipment - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Total Property, Plant and Equipment	$ 5,293,905	$ 5,250,805
Less: Accumulated Depreciation, Depletion and Amortization	3,323,580	3,272,255
Total Property, Plant and Equipment, Net	1,970,325	1,978,550
Coal and Other Plant and Equipment [Member]
Total Property, Plant and Equipment	3,252,874	3,213,512
Mining Properties and Mineral Rights [Member]
Total Property, Plant and Equipment	876,794	877,271
Airshafts [Member]
Total Property, Plant and Equipment	476,588	473,866
Mine Development [Member]
Total Property, Plant and Equipment	358,909	357,479
Coal Advance Mining Royalties [Member]
Total Property, Plant and Equipment	$ 328,740	$ 328,677