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Note 6 - Stock-based Compensation (Details Textual) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance 0   0  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period     35,880 43,540
Proceeds from Stock Options Exercised     $ 838,000 $ 1,018,000
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     $ 718,000 607,000
Benefit (Shortfall) of Tax Deductions from Share-based Compensation     $ 800,000 2,600,000
2005 Incentive Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 2,582,001   2,582,001  
Restricted Stock Units (RSUs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period     3 years  
Allocated Share-based Compensation Expense, Total $ 1,900,000 $ 1,300,000 $ 5,300,000 3,900,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total $ 11,800,000 $ 9,100,000 $ 11,800,000 $ 9,100,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition     1 year 328 days  
Performance-based Restricted Stock Units [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period     3 years