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Note 6 - Stock-based Compensation (Details Textual)
3 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
$ / shares
shares
Mar. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2017
shares
Dec. 31, 2016
shares
Dec. 31, 2015
shares
Dec. 31, 2014
shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance 0          
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period 35,880 18,040        
Proceeds from Stock Options Exercised | $ $ 838,000 $ 422,000        
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price | $ / shares $ 23.37          
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value | $ $ 718,000 262,000        
Benefit (Shortfall) of Tax Deductions from Share-based Compensation | $ $ 700,000 $ 2,600,000        
2005 Incentive Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 2,608,672          
Restricted Stock Units (RSUs) [Member]            
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period 122,674 87,781        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares $ 43.50 $ 38.59        
Number of Officers Receiving Award 6   6 7 7 7
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years          
Allocated Share-based Compensation Expense, Total | $ $ 1,500,000 $ 1,200,000        
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total | $ $ 15,100,000          
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 2 years 109 days          
Performance TSR Restricted Stock [Member]            
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period     23,556 30,319 61,209 60,456
Performance EPS Restricted Stock Units [Member]            
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period 55,455     58,241 57,409 57,642
Performance ROA Restricted Stock [Member]            
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period     22,377 29,119