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Note 13 - Outstanding Stock Options (Details) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 116,550 103,350 95,075 85,525
Exercise price, outstanding $ 37.42 $ 36.96 $ 35.11 $ 30.94
Remaining life 2 years 2 months 15 days      
Options exercisable 43,900      
Exercise price, exercisable $ 34.21      
Exercise Price $30.40 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 20,650      
Exercise price, outstanding $ 30.40      
Remaining life 1 month 17 days      
Options exercisable 20,650      
Exercise price, exercisable $ 30.40      
Exercise Price $36.50 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 25,000      
Exercise price, outstanding $ 36.50      
Remaining life 4 years 1 month 17 days      
Exercise Price $37.60 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 23,250      
Exercise price, outstanding $ 37.60      
Remaining life 1 year 1 month 17 days      
Options exercisable 23,250      
Exercise price, exercisable $ 37.60      
Exercise Price $38.15 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 23,925      
Exercise price, outstanding $ 38.15      
Remaining life 3 years 1 month 17 days      
Exercise Price $43.60 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 23,725      
Exercise price, outstanding $ 43.60      
Remaining life 2 years 1 month 20 days