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Note 15 - Stock-based Compensation (Details Textual) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 351,205    
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term 10 years    
Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Grant Date Fair Value $ 925,735 $ 632,255 $ 598,341
Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years    
Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 7 years    
The 2008 Equity Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 250,000    
2010 Equity Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 350,000    
The 2013 Equity Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 350,000    
The 2016 Equity Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 400,000    
Employee Stock Purchase Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 187,921    
Maximum Amount per Employee per Offering Period for Employee Stock Purchase Plan $ 10,000    
Maximum Amount per Employee per Offering Period for Employee Stock Purchase Plan, Percent 10.00% 8.00% 8.00%