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Income Taxes (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2012
Income Taxes      
Effective tax rate on income from continuing operations (as a percent) 25.00% 22.60%  
Maximum period for recognizing capital losses 5 years    
Valuation allowance $ 16   $ 16
State net operating losses 38    
Gross unrecognized tax benefit (expense) 117   116
Unrecognized tax benefits (net of federal tax benefits) that would impact the effective tax rate 38   38
Increase (reduction) in interest and penalties 1 (5)  
Accrued interest and penalties 37   36
Estimated decrease in amount of unrecognized tax benefits in next 12 months $ 76