Property, plant and equipment - Narrative (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 1,253	$ 1,135
Insurance and Other [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	477	426
Railroad, Utilities and Energy [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 732	$ 696