Note 3 - Property, Plant and Equipment - Depreciation and amortization (Details)		3 Months Ended	12 Months Ended
	Mar. 29, 2017 USD ($) a	Dec. 31, 2019 USD ($) area instrument	Dec. 31, 2019 USD ($) area instrument	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 03, 2020 USD ($) item
Property, plant, and equipment
Depreciation of plant and equipment			$ 14,219,000	$ 9,751,000	$ 2,618,000
Amortization of capitalized mine development costs			5,302,000	2,672,000	536,000
Total depreciation and amortization			19,521,000	12,423,000	3,154,000
Purchases of property, plant and equipment			$ 45,722,000	48,137,000	$ 75,039,000
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		$ 12,700,000	$ 12,700,000	$ 5,500,000
Area of Land | a	14,800
Purchases of property, plant and equipment	$ 125,000