Property, plant and equipment - Depreciation expense (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 5,541	$ 5,300
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	1,660	1,622
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 3,881	$ 3,678