Property, plant and equipment - Depreciation expense (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 1,968	$ 1,798
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	570	542
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 1,398	$ 1,256