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Note 12 - Quarterly Financial Information (Unaudited) (Tables)
 12 Months Ended
Dec. 31, 2016
Notes Tables  
Quarterly Financial Information [Table Text Block]
   
Quarter
  
   
First
    
Second
    
Third
   
Fourth
 
2016:
                                
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
  
                                 
2015:
                                
Operating revenues
   $
1,440,180
     $
1,539,957
     $
1,586,494
     $
1,621,015
  
Operating income
   $
155,220
     $
173,735
     $
193,846
     $
192,893
  
Net earnings
   $
91,932
     $
103,419
     $
115,139
     $
116,746
  
Basic earnings per share
   $
0.79
     $
0.89
     $
1.00
     $
1.02
  
Diluted earnings per share
   $
0.78
     $
0.88
     $
0.99
     $
1.01