Property, plant and equipment - Narrative (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 4,036	$ 3,814
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	1,497	1,443
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 2,403	$ 2,235