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Note 2 - Summary of Significant Accounting Policies (Details Textual) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities $ 0 $ 0 $ 0
Inventories Estimated Useful Lives 2 years    
Inventory, Finished Goods, Gross $ 1,900,000 2,400,000  
Depreciation 1,000,000 800,000 700,000
Impairment of Long-Lived Assets Held-for-use 0    
Sales Revenue, Services, Net 18,833,000 19,028,000 17,739,000
Advertising Expense $ 1,800,000 1,300,000 1,200,000
Number of Operating Segments 1    
Bill and Hold Arrangement [Member]      
Recognition of Deferred Revenue $ 938,000 $ 0 $ 0
Minimum [Member] | Retained Earnings [Member] | Accounting Standards Update 2014-09 [Member]      
Cumulative Effect of New Accounting Principle in Period of Adoption 4,800,000    
Maximum [Member] | Retained Earnings [Member] | Accounting Standards Update 2014-09 [Member]      
Cumulative Effect of New Accounting Principle in Period of Adoption $ 5,500,000    
Distributors [Member] | Minimum [Member]      
Standard Warranty Period 1 year 60 days    
Distributors [Member] | Maximum [Member]      
Standard Warranty Period 1 year 120 days    
Customer Concentration Risk [Member] | Accounts Receivable [Member]      
Concentration Risk, Percentage 12.00% 12.00%  
Geographic Concentration Risk [Member] | Sales Revenue, Net [Member] | UNITED STATES      
Concentration Risk, Percentage 62.00% 55.00% 52.00%
Geographic Concentration Risk [Member] | Sales Revenue, Net [Member] | Outside the United States [Member]      
Concentration Risk, Percentage 38.00% 45.00% 48.00%