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Note 7 - Stock-based Compensation (Details Textual) - USD ($)
3 Months Ended
Jan. 01, 2019
Jan. 01, 2018
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2014
Share-based Payment Arrangement, Expense     $ 780,000 $ 821,000    
The 2014 Employee Stock Purchase Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized     192,436     140,500
Share-based Compensation Arrangement by Share-based Payment Award, Number of Annual Additional Shares, Percentage         1.00%  
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Number of Additional Shares Authorizable 281,000          
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized   1        
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period     0 0    
Share-based Payment Arrangement, Expense     $ 50,900 $ 24,000