Properties (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Cost		$ 76,488	$ 75,420
Accumulated depreciation		22,126	21,259
Net book value		54,362	54,161
Land [Member]
Property, Plant and Equipment [Line Items]
Cost		5,283	5,246
Net book value		5,283	5,246
Road: Rail and Other Track Material [Member]
Property, Plant and Equipment [Line Items]
Cost		17,882	17,620
Accumulated depreciation		6,790	6,631
Net book value		$ 11,092	$ 10,989
Estimated useful life		43 years	42 years
Road: Ties [Member]
Property, Plant and Equipment [Line Items]
Cost		$ 11,292	$ 11,051
Accumulated depreciation		3,469	3,331
Net book value		$ 7,823	$ 7,720
Estimated useful life		34 years	34 years
Road: Ballast [Member]
Property, Plant and Equipment [Line Items]
Cost		$ 6,032	$ 5,926
Accumulated depreciation		1,828	1,753
Net book value		$ 4,204	$ 4,173
Estimated useful life		34 years	34 years
Road: Other Roadway [Member]
Property, Plant and Equipment [Line Items]
Cost	[1]	$ 21,294	$ 21,030
Accumulated depreciation	[1]	4,538	4,329
Net book value	[1]	$ 16,756	$ 16,701
Estimated useful life	[1]	49 years	48 years
Total Road [Member]
Property, Plant and Equipment [Line Items]
Cost		$ 56,500	$ 55,627
Accumulated depreciation		16,625	16,044
Net book value		39,875	39,583
Equipment: Locomotives [Member]
Property, Plant and Equipment [Line Items]
Cost		9,362	9,375
Accumulated depreciation		3,760	3,555
Net book value		$ 5,602	$ 5,820
Estimated useful life		17 years	17 years
Equipment: Freight Cars [Member]
Property, Plant and Equipment [Line Items]
Cost		$ 2,143	$ 2,118
Accumulated depreciation		815	789
Net book value		$ 1,328	$ 1,329
Estimated useful life		25 years	25 years
Equipment: Work Equipment and Other [Member]
Property, Plant and Equipment [Line Items]
Cost		$ 1,126	$ 1,107
Accumulated depreciation		392	351
Net book value		$ 734	$ 756
Estimated useful life		18 years	18 years
Total Equipment [Member]
Property, Plant and Equipment [Line Items]
Cost		$ 12,631	$ 12,600
Accumulated depreciation		4,967	4,695
Net book value		7,664	7,905
Technology and Other [Member]
Property, Plant and Equipment [Line Items]
Cost		1,205	1,199
Accumulated depreciation		534	520
Net book value		$ 671	$ 679
Estimated useful life		13 years	13 years
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Cost		$ 869	$ 748
Net book value		$ 869	$ 748

[1] Other roadway includes grading, bridges and tunnels, signals, buildings, and other road assets.