Note 6 - Income Taxes - Schedule of Reconciliation of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statutory U.S. federal income tax rate, amount	$ (1,941)	$ 20,600	$ 39,399
Statutory U.S. federal income tax rate, percent	21.00%	21.00%	21.00%
State income taxes, net of federal tax benefit, amount	$ (1,109)	$ 3,125	$ 3,240
State income taxes, net of federal tax benefit, percent	12.00%	3.20%	1.70%
Effect of foreign income taxes, amount	$ 406	$ 1,336	$ 28
Effect of foreign income taxes, percent	(4.40%)	1.40%	0.00%
Excess tax depletion, amount	$ 0	$ (13,141)	$ (20,873)
Excess tax depletion, percent	0.00%	(13.40%)	(11.10%)
Effect of change in U.S. tax law, amount	$ 0	$ 0	$ 2,777
Effect of change in U.S. tax law, percent	0.00%	0.00%	1.50%
Compensation, amount	$ 1,310	$ 1,799	$ 935
Compensation, percent	(14.20%)	1.80%	0.50%
Valuation allowance, amount	$ 1,479	$ 1,400	$ (1,379)
Valuation allowance, percent	(16.00%)	1.40%	(0.70%)
Tax credits, amount	$ 1,150	$ (2,536)	$ (980)
Tax credits, percent	(12.40%)	(2.60%)	(0.50%)
Non-controlling interest, amount	$ 726	$ (3,687)	$ (5,420)
Non-controlling interest, percent	(7.90%)	(3.80%)	(2.90%)
State rate change and prior period adjustments, amount	$ 1,797	$ (4,565)	$ (9,448)
State rate change and prior period adjustments, percent	(19.40%)	(4.60%)	(5.00%)
Other, amount	$ 154	$ 208	$ 549
Other, percent	(1.60%)	0.20%	0.30%
Total Income Tax Expense	$ 3,972	$ 4,539	$ 8,828
Income Tax Expense / Effective Rate, percent	(42.90%)	4.60%	4.80%