Note 6 - Income Taxes - Components of the Net Deferred Tax Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accrued reclamation costs	$ 32,938	$ 30,038
Deferred exploration	11,623	13,762
Foreign net operating losses	13,303	12,013
Domestic net operating losses	198,438	175,024
Pension and benefit obligation	12,341	13,866
Foreign exchange loss	19,808	21,719
Foreign tax credit carryforward	3,358	4,149
Miscellaneous	18,385	30,871
Total deferred tax assets	310,194	301,442
Valuation allowance	(77,210)	(86,634)	$ (94,981)	$ (78,684)
Total deferred tax assets	232,984	214,808
Miscellaneous	(2,551)	(904)
Properties, plants and equipment	(359,996)	(348,649)
Total deferred tax liabilities	(362,547)	(349,553)
Net deferred tax liability	$ (129,563)	$ (134,745)