Property, plant and equipment (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 43,867	$ 43,817
Property, plant and equipment, accumulated depreciation	(22,877)	(22,617)
Total property, plant and equipment, net	20,990	21,200
Insurance and Other [Member] | Land, buildings and improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	13,851	13,799
Insurance and Other [Member] | Machinery and equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	25,474	25,488
Insurance and Other [Member] | Furniture, fixtures and other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,542	4,530
Railroad [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	78,729	78,263
Property, plant and equipment, accumulated depreciation	(13,687)	(13,175)
Total property, plant and equipment, net	65,042	65,088
Railroad [Member] | Land, track structure and other roadway [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	64,278	63,824
Railroad [Member] | Locomotives, freight cars and other equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	13,596	13,523
Railroad [Member] | Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	855	916
Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	120,747	119,376
Property, plant and equipment, accumulated depreciation	(33,990)	(33,248)
Total property, plant and equipment, net	86,757	86,128
Utilities and Energy [Member] | Utility generation, transmission and distribution systems [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	87,898	86,730
Utilities and Energy [Member] | Interstate natural gas pipeline assets [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	16,712	16,667
Utilities and Energy [Member] | Independent power plants and other assets [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	12,828	12,671
Utilities and Energy [Member] | Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	3,309	3,308
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, net	$ 151,799	$ 151,216