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Marketable Securities (Narratives) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Jun. 30, 2012
D
M
Dec. 31, 2011
Marketable Securities    
Gross unrealized losses on marketable securities (pre-tax) $ 9 $ 12
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 4 4
Gross unrealized losses (pre-tax) within accumulated other comprehensive income related to auction rate securities 9 9
Minimum period that auction rate securities have been in a loss position (in months) 12  
Pre-determined intervals during which liquidity for auction rate securities is typically provided 1 (in days) 7  
Pre-determined intervals during which liquidity for auction rate securities is typically provided 2 (in days) 28  
Pre-determined intervals during which liquidity for auction rate securities is typically provided 3 (in days) 35  
Pre-determined intervals during which liquidity for auction rate securities is typically provided 4 (in days) 90  
Estimated fair value of current plus long term asset backed securities $ 813