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Consolidated Statement of Changes in Equity (Textuals) (Private Forward Repurchase Transaction [Member], USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended 3 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Mar. 31, 2013
Scenario, Forecast [Member]
Consolidated Statement of Changes in Equity and Comprehensive Income (Textuals) [Abstract]      
Private forward repurchase contract $ 200    
Common stock repurchased, shares 36,000,000 6,000,000 6,000,000