Property, plant and equipment - Depreciation expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 7,566	$ 7,284	$ 6,968
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	2,269	2,186	2,116
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 5,297	$ 5,098	$ 4,852