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Summary of significant accounting policies (Details) (USD $) In Billions	Jun. 30, 2011	Dec. 31, 2010
Summary of significant accounting policies (Textuals) [Abstract]
Accumulated depreciation and amortization of property, plant and equipment	$ 5.5	$ 5.2

- Definition

The cumulative amount of depreciation, depletion and amortization (related to property, plant and equipment, but not including land) that has been recognized in the income statement.

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