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Debt Obligations (Interest Expense Recognized on Convertible Senior Notes in Condensed Consolidated Statements of Income) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Debt Disclosure [Abstract]    
Accretion of debt discount $ 1 $ 1
Contractual interest 1 1
Total interest expense recognized on the 2013 Convertible Notes $ 2 $ 2