Note 16 - Coal Workers' Pneumoconiosis and Workers' Compensation - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Coal Workers Pneumoconiosis [Member]
Service cost	$ 4,603	$ 3,791	$ 6,650
Interest cost	6,206	7,001	5,245
Recognized net actuarial loss (gain)	5,604	1,016	(853)
State administrative fees and insurance bond premiums	0	0	0
Net periodic benefit costs	16,413	11,808	11,042
Workers Compensation [Member]
Service cost	6,276	5,685	6,230
Interest cost	1,844	2,585	2,283
Recognized net actuarial loss (gain)	(488)	(774)	(79)
State administrative fees and insurance bond premiums	1,996	2,157	2,671
Net periodic benefit costs	$ 9,628	$ 9,653	$ 11,105