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Note 4 - Concentration of Credit Risk (Details Textual) - Customer Concentration Risk [Member]
Pure in Thousands
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Revenue from Contract with Customer Benchmark [Member]      
Number of Major Customers 0 0 0
Accounts Receivable [Member]      
Number of Major Customers 0 0