Note 5 - Income Taxes - Reconciliation of Statutory Federal Income Tax Rate to Annual Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Computed “statutory” benefit (provision), Amount	$ 25,968	$ 6,986	$ 2,645
Computed “statutory” benefit (provision), Percentage	21.00%	21.00%	35.00%
Percentage depletion, Amount	$ 3,030	$ 3,158	$ 5,174
Percentage depletion, Percentage	2.00%	10.00%	68.00%
Change in valuation allowance, Amount	$ 686	$ (2,304)	$ 24,464
Change in valuation allowance, Percentage	1.00%	(7.00%)	324.00%
Federal rate change, Amount	$ 0	$ 0	$ (37,546)
Federal rate change, Percentage	0.00%	0.00%	(497.00%)
State taxes, net of federal taxes, Amount	$ 2,648	$ (849)	$ 1,112
State taxes, net of federal taxes, Percentage	2.00%	(3.00%)	15.00%
Foreign currency remeasurement of monetary assets and liabilities, Amount	$ (8,629)	$ 6,747	$ (12,812)
Foreign currency remeasurement of monetary assets and liabilities, Percentage	(7.00%)	20.00%	(169.00%)
Rate differential on foreign earnings, Amount	$ 2,705	$ (3,970)	$ (1,438)
Rate differential on foreign earnings, Percentage	2.00%	(12.00%)	(19.00%)
Compensation, Amount	$ (1,056)	$ (970)	$ (661)
Compensation, Percentage	(1.00%)	(3.00%)	(9.00%)
Foreign withholding taxes, Amount	$ 0	$ 278	$ (278)
Foreign withholding taxes, percentage	0.00%	1.00%	(4.00%)
Expiration of U.S. foreign tax credits, Amount	$ 0	$ 0	$ (2,300)
Expiration of U.S. foreign tax credits, percentage	0.00%	0.00%	(30.00%)
Other, Amount	$ (1,251)	$ (2,375)	$ 677
Other, Percentage	(1.00%)	(7.00%)	9.00%
Total benefit (provision)	$ 24,101	$ 6,701	$ (20,963)
Total benefit (provision)	19.00%	20.00%	(277.00%)

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