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Note 2 - Revenue Recognition (Details Textual) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Lapse Period for Consignment 60 days    
Accounts Receivable, after Allowance for Credit Loss, Current, Total $ 58,889   $ 55,214
Accounts Receivable, Allowance for Credit Loss, Writeoff 0   0
Contract with Customer, Liability, Current $ 2,927   $ 2,520
Revenue, Performance Obligation, Performance Period Following Prepayment 60 days    
Minimum [Member]      
Contract with Customer, Payment Term 30 days    
Maximum [Member]      
Contract with Customer, Payment Term 60 days    
Certain Customers in China [Member]      
Contract with Customer, Payment Term Required Prior to Shipping Products     14 days
Contract with Customer, Liability, Revenue Recognized $ 2,400    
Certain Customers in China [Member] | Accrued Liabilities, Current [Member]      
Contract with Customer, Liability, Current $ 2,900   $ 2,500
Customer Concentration Risk [Member] | Revenue Benchmark [Member] | Distributor Sales [Member]      
Concentration Risk, Percentage 84.00% 88.00%  
Product [Member]      
Percentage of Revenue 99.00% 97.00%