Property, plant and equipment - Depreciation expense (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 1,798	$ 1,760
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	542	535
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 1,256	$ 1,225