Note 6 - Income Taxes - Reconciliation of Statutory Federal Income Tax Rate to Annual Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Computed “statutory” benefit (provision), Amount	$ 6,986	$ 2,645	$ (31,381)
Computed “statutory” benefit (provision), Percentage	(21.00%)
Computed “statutory” benefit (provision), Percentage		35.00%	35.00%
Percentage depletion, Amount	$ 3,158	$ 5,174	$ 8,114
Percentage depletion, Percentage	(10.00%)	68.00%	(9.00%)
Change in valuation allowance, Amount	$ (2,304)	$ 24,464	$ 11,336
Change in valuation allowance, Percentage	7.00%	324.00%	(13.00%)
Federal rate change, Amount		$ (37,546)
Federal rate change, Percentage		(497.00%)
State taxes, net of federal taxes, Amount	$ (849)	$ 1,112	$ (565)
State taxes, net of federal taxes, Percentage	3.00%	15.00%
Foreign currency remeasurement of monetary assets and liabilities, Amount	$ 6,747	$ (12,812)	$ (7,820)
Foreign currency remeasurement of monetary assets and liabilities, Percentage	(20.00%)	(169.00%)	9.00%
Rate differential on foreign earnings, Amount	$ (3,970)	$ (1,438)	$ (6,853)
Rate differential on foreign earnings, Percentage	12.00%	(19.00%)	8.00%
Compensation, Amount	$ (970)	$ (661)	$ (1,517)
Compensation, Percentage	3.00%	(9.00%)	2.00%
Foreign withholding taxes, Amount	$ 278	$ (278)
Foreign withholding taxes, percentage	(1.00%)	(4.00%)
Expiration of U.S. foreign tax credits, Amount		$ (2,300)
Expiration of U.S. foreign tax credits, percentage		(30.00%)
Other, Amount	$ (2,375)	$ 677	$ 596
Other, Percentage	7.00%	9.00%	(1.00%)
Total benefit (provision)	$ 6,701	$ (20,963)	$ (28,090)
Total benefit (provision)	(20.00%)	(277.00%)	31.00%

Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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