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Income taxes (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Jun. 30, 2011
Jun. 30, 2010
Income taxes.        
Income tax expense $ 134 $ 82 $ 308 $ 212
Effective income tax rate (percent in hundredths) 28.60%   26.90% 26.10%
Statutory income tax rate (percent in hundredths) 35.00%