Property, plant and equipment (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 43,009	$ 42,210
Property, plant and equipment, accumulated depreciation	(21,868)	(20,772)
Total property, plant and equipment, net	21,141	21,438
Insurance and Other [Member] | Land, buildings and improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	13,555	13,259
Insurance and Other [Member] | Machinery and equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	24,765	24,285
Insurance and Other [Member] | Furniture, fixtures and other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,689	4,666
Railroad [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	77,870	76,634
Property, plant and equipment, accumulated depreciation	(12,724)	(12,101)
Total property, plant and equipment, net	65,146	64,533
Railroad [Member] | Land, track structure and other roadway [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	63,318	62,404
Railroad [Member] | Locomotives, freight cars and other equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	13,477	13,482
Railroad [Member] | Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,075	748
Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	105,108	101,841
Property, plant and equipment, accumulated depreciation	(29,856)	(28,536)
Total property, plant and equipment, net	75,252	73,305
Utilities and Energy [Member] | Utility generation, transmission and distribution systems [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	82,743	81,127
Utilities and Energy [Member] | Interstate natural gas pipeline assets [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	8,281	8,165
Utilities and Energy [Member] | Independent power plants and other assets [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	8,952	8,817
Utilities and Energy [Member] | Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	5,132	3,732
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, net	$ 140,398	$ 137,838