Note 6 - Income Taxes - Reconciliation of Statutory Federal Income Tax Rate to Annual Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Computed “statutory” benefit (provision), Amount	$ 3,498	$ 25,968	$ 6,986
Computed “statutory” benefit (provision), Percentage	21.00%	21.00%	21.00%
Percentage depletion, Amount	$ 5,327	$ 3,030	$ 3,158
Percentage depletion, Percentage	32.00%	2.00%	10.00%
Change in valuation allowance, Amount	$ 786	$ 686	$ (2,304)
Change in valuation allowance, Percentage	5.00%	1.00%	(7.00%)
State taxes, net of federal tax benefit, Amount	$ (1,164)	$ 2,648	$ (849)
State taxes, net of federal tax benefit, Percentage	(7.00%)	2.00%	(3.00%)
Foreign currency remeasurement of monetary assets and liabilities, Amount	$ (4,824)	$ (8,629)	$ 6,747
Foreign currency remeasurement of monetary assets and liabilities, Percentage	(29.00%)	(7.00%)	20.00%
Rate differential on foreign earnings, Amount	$ 991	$ 3,947	$ (469)
Rate differential on foreign earnings, Percentage	6.00%	3.00%	(1.00%)
Compensation, Amount	$ (458)	$ (1,056)	$ (970)
Compensation, Percentage	(3.00%)	(1.00%)	(3.00%)
Foreign withholding taxes, Amount	$ 0	$ 0	$ 278
Foreign withholding taxes, percentage	0.00%	0.00%	1.00%
Mining and other taxes, Amount	$ (3,572)	$ (1,242)	$ (3,501)
Mining and other taxes, percentage	(22.00%)	(1.00%)	(11.00%)
Other, Amount	$ (719)	$ (1,251)	$ (2,375)
Other, Percentage	(4.00%)	(1.00%)	(7.00%)
Total (provision) benefit	$ (135)	$ 24,101	$ 6,701
Total (provision) benefit	(1.00%)	19.00%	20.00%

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to mining and other taxes.

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

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Percentage of the difference, between reported income tax expense (benefit) and the expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations, that is attributable to tax exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority interest income (expense), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, change in enacted tax rate, prior year income taxes, change in deferred tax asset valuation allowance, and other adjustments.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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Amount of reported income tax expense (benefit) in excess of (less than) expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operation, attributable to nondeductible expense for award under share-based payment arrangement. Includes, but is not limited to, expense determined to be nondeductible upon grant or after for award under share-based payment arrangement.

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