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Note 6 - Stock-based Compensation (Details Textual) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period   35,880  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance   0  
Proceeds from Stock Options Exercised $ 838  
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price   $ 23.37  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value   $ 718  
Benefit (Shortfall) of Tax Deductions from Share-based Compensation $ 500 700  
2005 Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 2,605,454    
Minimum [Member]      
Performance-based Restricted Stock Units, Number of Shares That May Vest, Percentage of Target 0.00%    
Maximum [Member]      
Performance-based Restricted Stock Units, Number of Shares That May Vest, Percentage of Target 150.00%    
Restricted Stock Units (RSUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years    
Share-based Payment Arrangement, Expense $ 1,500 $ 1,500  
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total $ 8,600   $ 15,100
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition 1 year 219 days    
Performance-based Restricted Stock Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years