Note 4 - Property, Plant and Equipment - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property, plant and equipment, net	$ 170,431	$ 161,662
Computer Equipment [Member]
Property, plant and equipment, gross	487	487
Furniture and Fixtures [Member]
Property, plant and equipment, gross	464	464
Leasehold Improvements [Member]
Property, plant and equipment, gross	101	101
Excluding Construction in Progress [Member]
Property, plant and equipment, gross	1,052	1,052
Less: accumulated depreciation	(797)	(660)
Property, plant and equipment, net	255	392
Construction in Progress [Member]
Property, plant and equipment, gross	170,176	161,270
Construction in Progress [Member] \| Rio Grande [Member]
Property, plant and equipment, gross	149,160	140,253
Construction in Progress [Member] \| Rio Bravo [Member]
Property, plant and equipment, gross	$ 21,016	$ 21,017

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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