Note 16 - Coal Workers' Pneumoconiosis and Workers' Compensation - Schedule of Weighted-Average Assumptions Used (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Coal Workers Pneumoconiosis [Member]
Discount rate assumption used to determine net periodic benefit costs	2.53%	3.41%	4.42%
Discount rate	3.41%	4.42%	3.75%
Workers Compensation [Member]
Discount rate assumption used to determine net periodic benefit costs	2.35%	3.25%	4.26%
Discount rate	3.25%	4.26%	3.57%