Note 2 - Long-lived Asset Impairments (Details Textual) $ in Thousands	1 Months Ended		3 Months Ended				12 Months Ended
	Aug. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Asset Impairment Charges, Total							$ 1,799	$ 77,882
Carlisle Mine [Member]
Asset Impairment Charges, Total						$ 65,700	$ 0
Mineral Properties, Net, Total		$ 1,800				$ 1,800		$ 1,800
Equipment, Redeployed					$ 23,000
Carlisle Mine [Member] | Closing of Mine [Member]
Restructuring and Related Costs, Incurred Cost, Total				$ 1,100	$ 1,100
Carlisle Mine [Member] | Measurement Input, Discount Rate [Member] | Valuation Technique, Discounted Cash Flow [Member]
Asset Impairment, Measurement Input		0.10				0.10		0.10
Bulldog Reserves [Member]
Asset Impairment Charges, Total						$ 9,200
Hourglass Sands [Member]
Asset Impairment Charges, Total	$ 1,800	$ 2,900	$ 1,800					$ 2,900
Mineral Properties, Net, Total		$ 1,900				$ 1,900		$ 1,900