Note 1 - Summary of Significant Accounting Policies (Details Textual) $ in Thousands	12 Months Ended
Dec. 31, 2021 USD ($) T	Dec. 31, 2020 USD ($)
Accounts Receivable, Allowance for Credit Loss, Ending Balance \| $	$ 0	$ 0
Regulatory Assets, Total \| $	$ 23,500
Coal Supply Commitment (US Ton) \| T	20.7
Year Supply Commitments End	2027
Priced Coal Supply Commitment (US Ton) \| T	16.2
Revenue, Product and Service Benchmark [Member] \| Customer Concentration Risk [Member] \| Coal [Member]
Number of Major Customers	5	4
Revenue, Product and Service Benchmark [Member] \| Customer Concentration Risk [Member] \| Five Customers [Member] \| Coal [Member]
Concentration Risk, Percentage	95.00%
Revenue, Product and Service Benchmark [Member] \| Customer Concentration Risk [Member] \| Four Customers [Member] \| Coal [Member]
Concentration Risk, Percentage		79.00%
Accounts Receivable [Member] \| Customer Concentration Risk [Member] \| Coal [Member]
Number of Major Customers	5	4
Accounts Receivable [Member] \| Customer Concentration Risk [Member] \| Five Customers [Member] \| Coal [Member]
Concentration Risk, Percentage	99.00%
Accounts Receivable [Member] \| Customer Concentration Risk [Member] \| Four Customers [Member] \| Coal [Member]
Concentration Risk, Percentage		87.00%
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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Represents the number of underground mines that are included in the significant operating segment.

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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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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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