Note 2 - Long-lived Asset Impairments (Details Textual) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2019	Jun. 30, 2020
Carlisle Mine [Member]
Asset Impairment Charges, Total	$ 0	$ 65,700
Mineral Properties, Net, Total				$ 1,800
Equipment, Redeployed	23,000
Carlisle Mine [Member] | Closing of Mine [Member]
Restructuring and Related Cost, Expected Cost, Total				1,100
Restructuring and Related Costs, Incurred Cost, Total	$ 600
Bulldog Reserves [Member]
Asset Impairment Charges, Total		9,200
Hourglass Sands [Member]
Asset Impairment Charges, Total			$ 2,900
Mineral Properties, Net, Total		$ 1,900	$ 1,900	$ 1,800