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Properties - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Property, Plant and Equipment [Line Items]      
Percentage of labor costs relating to the deconstruction of old track 20.00%    
Percentage of labor costs relating to the installation of new track 80.00%    
Percentage of assets depreciated under the group-life method 87.00%    
Properties $ 45,530 $ 45,100 [1]  
Depreciation 1,383 1,349 [1] $ 1,331 [1]
Gains on sale of properties 35 151 154
Impairment expense 8 22 $ 24
Total Equipment      
Property, Plant and Equipment [Line Items]      
Properties $ 10,227 10,708  
Life studies, frequency period 3 years    
Total Road      
Property, Plant and Equipment [Line Items]      
Properties $ 33,109 $ 32,125  
Life studies, frequency period 6 years    
[1] Certain prior year data has been reclassified to conform to the current presentation.