Property, plant and equipment - Narrative (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 2,518	$ 2,276
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	955	791
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 1,476	$ 1,396