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Note 14 - Quarterly Financial Information (Unaudited) (Tables)
 12 Months Ended
Dec. 31, 2017
Notes Tables  
Quarterly Financial Information [Table Text Block]
   
Quarter
  
   
First
    
Second
    
Third
    
Fourth
  
201
7:
                                
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
  
                                 
201
6:
                                
Operating revenues
   $
1,528,712
     $
1,615,026
     $
1,690,659
     $
1,721,062
  
Operating income
   $
167,890
     $
175,792
     $
182,977
     $
194,361
  
Net earnings
   $
100,098
     $
105,011
     $
109,425
     $
117,556
  
Basic earnings per share
   $
0.89
     $
0.93
     $
0.97
     $
1.05
  
Diluted earnings per share
   $
0.88
     $
0.92
     $
0.97
     $
1.05