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Note 13 - Outstanding Stock Options (Details) - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 103,350 95,075 85,525 73,950
Exercise price, outstanding $ 36.96 $ 35.11 $ 30.94 $ 27.14
Remaining life 2 years 5 months 19 days      
Options exercisable 30,200      
Exercise price, exercisable $ 30.08      
Exercise Price $29.25 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 8,400      
Exercise price, outstanding $ 29.25      
Remaining life 29 days      
Options exercisable 8,400      
Exercise price, exercisable $ 29.25      
Exercise Price $30.40 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 21,800      
Exercise price, outstanding $ 30.40      
Remaining life 1 year 1 month 17 days      
Options exercisable 21,800      
Exercise price, exercisable $ 30.40      
Exercise Price $37.60 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 23,950      
Exercise price, outstanding $ 37.60      
Remaining life 2 years 1 month 17 days      
Exercise Price $43.60 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 24,500      
Exercise price, outstanding $ 43.60      
Remaining life 3 years 1 month 20 days      
Exercise Price $38.15 [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Options outstanding 24,700      
Exercise price, outstanding $ 38.15      
Remaining life 4 years 1 month 17 days