Note 16 - Coal Workers' Pneumoconiosis and Workers' Compensation - Schedule of Changes in Benefit Obligation, Plan Assets and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Coal Workers Pneumoconiosis [Member]
Benefit obligation at beginning of period	$ 214,473	$ 177,188
State administrative fees and insurance bond premiums	0	0	$ 0
Service cost	4,603	3,791	6,650
Interest cost	6,206	7,001	5,245
Actuarial loss (gain)	29,510	39,827
Benefits and other payments	(12,869)	(13,334)
Benefit obligation at end of period	241,923	214,473	177,188
Current assets	0	0
Current liabilities	(12,203)	(12,331)
Noncurrent liabilities	(229,720)	(202,142)
Net obligation recognized	(241,923)	(214,473)
Net actuarial loss	71,259	47,352
Net amount recognized (before tax effect)	71,259	47,352
Workers Compensation [Member]
Benefit obligation at beginning of period	71,480	70,986
State administrative fees and insurance bond premiums	1,996	2,157	2,671
Service cost	6,276	5,685	6,230
Interest cost	1,844	2,585	2,283
Actuarial loss (gain)	1,897	1,536
Benefits and other payments	(10,052)	(11,469)
Benefit obligation at end of period	73,441	71,480	$ 70,986
Current assets	602	1,037
Current liabilities	(9,653)	(11,323)
Noncurrent liabilities	(64,390)	(61,194)
Net obligation recognized	(73,441)	(71,480)
Net actuarial loss	(8,866)	(11,250)
Net amount recognized (before tax effect)	$ (8,866)	$ (11,250)