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Note 14 - Quarterly Financial Information (Unaudited) (Tables)
 12 Months Ended
Dec. 31, 2018
Notes Tables  
Quarterly Financial Information [Table Text Block]
   
Quarter
  
   
First
    
Second
    
Third
    
Fourth
  
2018:
                                
Operating revenues
   $
1,948,245
     $
2,139,027
     $
2,209,760
     $
2,317,842
  
Operating income
   $
168,781
     $
214,812
     $
174,688
     $
122,740
  
Net earnings
   $
118,142
     $
151,652
     $
131,110
     $
88,681
  
Basic earnings per share
   $
1.08
     $
1.39
     $
1.20
     $
0.81
  
Diluted earnings per share
   $
1.07
     $
1.37
     $
1.19
     $
0.81
  
                                 
2017:
                                
Operating revenues
   $
1,629,158
     $
1,726,915
     $
1,843,334
     $
1,990,160
  
Operating income
   $
149,389
     $
163,615
     $
164,972
     $
145,814
  
Net earnings
   $
102,702
     $
97,869
     $
100,385
     $
385,308
  
Basic earnings per share
   $
0.93
     $
0.89
     $
0.92
     $
3.51
  
Diluted earnings per share
   $
0.92
     $
0.88
     $
0.91
     $
3.48