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Available-For-Sale Investments (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Available-For-Sale Investments [Abstract]      
Impairment loss for securities that have credit losses $ 9 $ 18  
Proceeds from sales and maturities of short term investments $ 2,269 $ 3,171 $ 2,061