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Marketable Securities (Narratives) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Marketable Securities    
Gross unrealized losses on marketable securities (pre-tax) $ 6 $ 6
Gross unrealized gains on marketable securities (pre-tax) 2 3
Percentage of interests in auction rate securities to portfolio, maximum (as a percent) 1.00%  
Estimated fair value of auction-rate securities 7 7
Gross unrealized losses (pre-tax) within accumulated other comprehensive income related to auction rate securities 6 6
Minimum period that auction rate securities have been in a loss position 12 months  
Pre-determined intervals during which liquidity for auction rate securities is typically provided 1 7 days  
Pre-determined intervals during which liquidity for auction rate securities is typically provided 2 28 days  
Pre-determined intervals during which liquidity for auction rate securities is typically provided 3 35 days  
Pre-determined intervals during which liquidity for auction rate securities is typically provided 4 90 days  
Number of asset-backed securities rated AA/Aa2 1  
Fair market value of asset-backed securities rated AA/Aa2 4  
Estimated fair value of current plus long term asset backed securities $ 1,252