Note 9 - Property, Plant and Equipment - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Total Property, Plant and Equipment	$ 5,333,733	$ 5,250,805
Less: Accumulated Depreciation, Depletion and Amortization	3,373,966	3,272,255
Total Property, Plant and Equipment, Net	1,959,767	1,978,550
Coal and Other Plant and Equipment [Member]
Total Property, Plant and Equipment	3,279,307	3,209,232
Mining Properties and Mineral Rights [Member]
Total Property, Plant and Equipment	884,597	881,551
Airshafts [Member]
Total Property, Plant and Equipment	480,620	473,866
Mine Development [Member]
Total Property, Plant and Equipment	360,225	357,479
Coal Advance Mining Royalties [Member]
Total Property, Plant and Equipment	$ 328,984	$ 328,677