Note 2 - Long-lived Asset Impairments (Details Textual) - Hourglass Sands [Member] - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
Aug. 31, 2020	Dec. 31, 2019	Sep. 30, 2020	Dec. 31, 2019
Asset Impairment Charges, Total	$ 1.8	$ 2.9	$ 1.8	$ 2.9
Mineral Properties, Net, Total		$ 1.9		$ 1.9

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2420-110228

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration