Property, plant and equipment - Depreciation expense (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 5,830	$ 5,541
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	1,725	1,660
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 4,105	$ 3,881