Investment gains/losses - Narrative (Detail) (USD $) In Millions	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended
Sep. 30, 2011 Goldman Sachs Group Inc [Member] Cumulative Preferred Stock [Member] Equity Securities and Other Investments [Member]	Sep. 30, 2011 Equity Securities and Other Investments [Member]	Sep. 30, 2010 Equity Securities and Other Investments [Member]	Sep. 30, 2011 Equity Securities and Other Investments [Member]	Sep. 30, 2010 Equity Securities and Other Investments [Member]	Sep. 30, 2010 BNSF [Member] Other Investment Gains / Losses [Member]	Mar. 31, 2010 BNSF [Member]
Investment gains/losses [Line Items]
Gains from sales and other disposals	$ 1,250	$ 40	$ 522	$ 1,308	$ 857
One-time holding gain related to the BNSF acquisition						$ 979	$ 1,000

In a business combination achieved in stages, this element represents the amount of net gain (loss) recognized by the entity as a result of remeasuring to fair value the equity interest in the acquiree it held before the business combination.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Number 141R

-Paragraph 68

-Name Statement of Financial Accounting Standard (FAS)

-Subparagraph q(2)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 48

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (g)(2)

-URI http://asc.fasb.org/extlink&oid=6909870&loc=d3e1392-128463

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