Note 3 - Property, Plant and Equipment - Depreciation and amortization (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($) area instrument	Dec. 31, 2020 USD ($) area instrument	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 03, 2020 USD ($) item
Property, plant, and equipment
Depreciation of plant and equipment		$ 17,094,000	$ 14,219,000	$ 9,751,000
Amortization of capitalized mine development costs		3,818,000	5,302,000	2,672,000
Total depreciation and amortization		20,912,000	19,521,000	12,423,000
Purchases of property, plant and equipment		$ 24,753,000	45,722,000	$ 48,137,000
Knox Creek preparation plant
Property, plant, and equipment
Consideration for acquiring permits	$ 600,000
Number of lease instruments | instrument	2	2
Minimum consideration recoupable against future royalty payments	$ 100,000
Number of separate mining areas | area	2	2
Property Adjacent To Elk Creek complex
Property, plant, and equipment
Proven and probable reserves					$ 21,000,000
Number of different coals In reserve base | item					20
Coal Properties
Property, plant, and equipment
Capitalized Costs Related to Coal Reserves	$ 15,400,000	$ 15,400,000	$ 12,700,000