Property, plant and equipment (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 43,817	$ 42,210
Property, plant and equipment, accumulated depreciation	(22,617)	(20,772)
Total property, plant and equipment, net	21,200	21,438
Insurance and Other [Member] | Land, buildings and improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	13,799	13,259
Insurance and Other [Member] | Machinery and equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	25,488	24,285
Insurance and Other [Member] | Furniture, fixtures and other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,530	4,666
Railroad [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	78,263	76,634
Property, plant and equipment, accumulated depreciation	(13,175)	(12,101)
Total property, plant and equipment, net	65,088	64,533
Railroad [Member] | Land, track structure and other roadway [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	63,824	62,404
Railroad [Member] | Locomotives, freight cars and other equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	13,523	13,482
Railroad [Member] | Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	916	748
Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	119,376	101,841
Property, plant and equipment, accumulated depreciation	(33,248)	(28,536)
Total property, plant and equipment, net	86,128	73,305
Utilities and Energy [Member] | Utility generation, transmission and distribution systems [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	86,730	81,127
Utilities and Energy [Member] | Interstate natural gas pipeline assets [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	16,667	8,165
Utilities and Energy [Member] | Independent power plants and other assets [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	12,671	8,817
Utilities and Energy [Member] | Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	3,308	3,732
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, net	$ 151,216	$ 137,838