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Note 1 - Nature of Operations (Details Textual)
$ in Millions
 12 Months Ended
Dec. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Network Route Miles 32,452    
Route Miles Owned 26,124    
Number of Reportable Segments 1    
Number of Operating Segments 1    
Revenue from Contract with Customer, Excluding Assessed Tax, Total $ 21,804 $ 19,533 $ 21,708
MEXICO      
Revenue from Contract with Customer, Excluding Assessed Tax, Total $ 2,400 $ 2,100 $ 2,300