Property, plant and equipment - Depreciation expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 8,119	$ 7,566	$ 7,284
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	2,320	2,269	2,186
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 5,799	$ 5,297	$ 5,098