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Properties - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
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%    
Total fixed assets   $ 45,100 $ 44,805  
Depreciation expense   1,349 1,331 $ 1,315
Gains on sale of properties   151 154 18
Impairment expense   22 24 $ 25
Total Equipment        
Property, Plant and Equipment [Line Items]        
Total fixed assets   $ 10,708 11,092  
Life studies, frequency period   3 years    
Total Equipment | Service Life | Forecast        
Property, Plant and Equipment [Line Items]        
Depreciation expense $ 30      
Total Road        
Property, Plant and Equipment [Line Items]        
Total fixed assets   $ 32,125 $ 31,411  
Life studies, frequency period   6 years