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Note 1 - Description of Business (Details)
$ in Millions
12 Months Ended
Dec. 31, 2020
customer
Dec. 31, 2021
USD ($)
item
Oct. 29, 2021
USD ($)
Number of primary properties | item   4  
Number of mines in operation | item   3  
Revenue Benchmark      
Number of Major Customers | customer 3    
Revolving Credit Facility      
Outstanding on Revolving Credit Facility | $   $ 0.0  
Revolving Credit Facility | KeyBank National Association      
Aggregate principal amount | $     $ 30.0