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Note 13 - Stock Option Plan (Details) - Outstanding Stock Options - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 85,525 73,950 82,875 88,375
Exercise price, outstanding $ 30.94 $ 27.14 $ 24.33 $ 23.27
Remaining life 2 years 7 months 28 days      
Options exercisable 19,175      
Exercise price, exercisable $ 24.48      
Exercise Price $23.20 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 6,650      
Exercise price, outstanding $ 23.20      
Remaining life 21 days      
Options exercisable 6,650      
Exercise price, exercisable $ 23.20      
Exercise Price $25.16 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 12,525      
Exercise price, outstanding $ 25.16      
Remaining life 1 year 29 days      
Options exercisable 12,525      
Exercise price, exercisable $ 25.16      
Exercise Price $29.25 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 17,850      
Exercise price, outstanding $ 29.25      
Remaining life 2 years 29 days      
Exercise Price $30.40 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 23,500      
Exercise price, outstanding $ 30.40      
Remaining life 3 years 1 month 17 days      
Exercise Price $37.60 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 25,000      
Exercise price, outstanding $ 37.60      
Remaining life 4 years 1 month 17 days