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Income taxes (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Income taxes.    
Income tax expense$ 17$ 18$ 325$ 229
Effective income tax rate (percent in hundredths)10.70% 24.80%26.00%
Statutory income tax rate (percent in hundredths)35.00%