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Note 10 - Income Taxes (Tables)
6 Months Ended
Jun. 30, 2018
Notes Tables  
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]
   
Three Months Ended
   
Six Months Ended
 
   
June 30,
   
June 30,
 
   
2018
   
2017
   
2018
   
2017
 
Income tax expense
  $
14,856
    $
47,893
    $
21,256
    $
55,222
 
Effective income tax rate
   
26.7
%    
40.4
%    
26.0
%    
40.5
%