Property, plant and equipment - Depreciation expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 7,284	$ 6,968	$ 6,656
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	2,186	2,116	2,017
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 5,098	$ 4,852	$ 4,639