Note 1 - Summary of Significant Accounting Policies (Details Textual) $ in Thousands, T in Millions	12 Months Ended
Dec. 31, 2020 USD ($) T	Dec. 31, 2019 USD ($)
Accounts Receivable, Allowance for Credit Loss, Ending Balance \| $	$ 0	$ 0
Surety bonds \| $	$ 27,000
Coal Supply Commitment (US Ton) \| T	21.6
Year Supply Commitments End	2027
Priced Coal Supply Commitment (US Ton) \| T	13.7
Revenue, Product and Service Benchmark [Member] \| Customer Concentration Risk [Member] \| Coal [Member]
Number of Major Customers	4	4
Revenue, Product and Service Benchmark [Member] \| Customer Concentration Risk [Member] \| Four Customers [Member] \| Coal [Member]
Concentration Risk, Percentage	79.00%	70.00%
Accounts Receivable [Member] \| Customer Concentration Risk [Member] \| Coal [Member]
Number of Major Customers	4	3
Accounts Receivable [Member] \| Customer Concentration Risk [Member] \| Four Customers [Member] \| Coal [Member]
Concentration Risk, Percentage	87.00%
Accounts Receivable [Member] \| Customer Concentration Risk [Member] \| Three Customers [Member] \| Coal [Member]
Concentration Risk, Percentage		68.00%
Minimum [Member]
Property, Plant and Equipment, Useful Life (Year)	3 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)	2 years
Minimum [Member] \| Measurement Input, Discount Rate [Member]
ARO measurement input	0.050
Maximum [Member]
Property, Plant and Equipment, Useful Life (Year)	25 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)	4 years
Maximum [Member] \| Measurement Input, Discount Rate [Member]
ARO measurement input	0.10
Southwestern Indiana [Member]
Number of Underground Mines Included in the Significant Operating Segment	2

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Represents the number of major customers accounting for 10% or more of the specified concentration risk benchmark, which includes, but not limited to, sales revenue, accounts receivable, etc.

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Amount of allowance for credit loss on accounts receivable.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Value of input used to measure debt instrument, including, but not limited to, convertible and non-convertible debt.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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