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Income taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Mar. 31, 2012
Income Taxes [Line Items]
Tax reserves	$ 263		$ 261
Impact on tax expense if tax reserves were unnecessary	263
Accrued interest, related to income taxes in the balance sheet	67
Additional tax expense related to interest	8	5
Reasonably possible increase in uncertain tax positions within the next 12 months, if a re-evaluation is required	930
United Kingdom
Income Taxes [Line Items]
The total amount of disputed tax assessments for the life of a transaction, covering the period from 2001 - 2006 related to foreign tax credits	$ 900

This element represents the amount of interest expense accrued as of the date of the statement of financial position for an underpayment of income taxes computed by applying the applicable statutory rate of interest to the difference between a tax position recognized for financial reporting purposes and the amount previously taken or expected to be taken in a tax return of the entity.

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- Definition

This element represents interest expense recognized for an underpayment of income taxes computed by applying the applicable statutory rate of interest to the difference between a tax position recognized for financial reporting purposes and the amount previously taken or expected to be taken in a tax return of the entity.

+ References+ Details