Property, plant and equipment - Depreciation expense (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 4,138	$ 3,863
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	1,148	1,147
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 2,990	$ 2,716