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Note 9 - Stock-based Compensation Expense (Details Textual) - USD ($)
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Share-based Payment Arrangement, Expense, Par Value $ 44,000 $ 73,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance (in shares) 410,000 417,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number (in shares) 370,000 292,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period (in shares) 0  
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total $ 7,900  
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year) 1 year  
Restricted Stock Units (RSUs) [Member]    
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total $ 20,000  
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year) 1 year  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value $ 29,700