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Income Taxes Income Taxes (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Income Tax Contingency [Line Items]    
Income tax expense (benefit) $ 120 $ (30)
Income before income tax expense (benefit) $ 815 $ 71
Effective income tax rate 14.70% (42.30%)
Federal stautory income tax rate 35.00%