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Note 6 - Stock-based Compensation (Tables)
 9 Months Ended
Sep. 30, 2018
Notes Tables  
Schedule of Share-based Payment Award, Equity Awards Other than Options, Valuation Assumptions [Table Text Block]
Risk-free interest rate
    
2.84
 %
Expected life (in years)
    
5.5
  
Dividend yield
    
0.79
 %
Expected volatility
    
25.24
%
Forfeiture rate
    
0.0
 %
Sub-Optimal Exercise Factor
    
2.5
  
Risk-free interest rate
    
2.79
 %
Expected life (in years)
    
6
  
Dividend yield
    
0.92
 %
Expected volatility
    
25.64
%
Forfeiture rate for employees
    
2.78
 %
Forfeiture rate for members of the senior management
    
0.0
 %
Sub-Optimal Exercise Factor for employees
    
2.0
  
Sub-Optimal Exercise Factor for members of the senior management
    
2.8