Property, plant and equipment - Depreciation expense (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 3,636	$ 3,524
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	1,092	1,080
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 2,544	$ 2,444