Property, plant and equipment - Depreciation expense (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 6,210	$ 5,830
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	1,725	1,725
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 4,485	$ 4,105