Property, plant and equipment - Depreciation expense (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 3,863	$ 3,636
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	1,147	1,092
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 2,716	$ 2,544