Note 2 - Long-lived Asset Impairments (Details Textual) - USD ($) $ in Thousands	3 Months Ended					9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Asset Impairment Charges, Total	$ 1,799				$ 0	$ 1,799	$ 0
Carlisle Mine [Member]
Asset Impairment Charges, Total			$ 0	$ 65,700
Mineral Properties, Net, Total	1,800			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
Bulldog Reserves [Member]
Asset Impairment Charges, Total				$ 9,200
Hourglass Sands [Member]
Asset Impairment Charges, Total	1,800							$ 2,900
Mineral Properties, Net, Total	$ 1,900					$ 1,900