Property, plant and equipment - Narrative (Detail) (USD $) In Millions	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010
Property, Plant, and Equipment Disclosure [Line Items]
Depreciation expense	$ 2,276	$ 2,015
Insurance and Other [Member]
Property, Plant, and Equipment Disclosure [Line Items]
Depreciation expense	791	762
Railroad, Utilities and Energy [Member]
Property, Plant, and Equipment Disclosure [Line Items]
Depreciation expense	$ 1,396	$ 1,144