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Marketable Securities (Tables)
6 Months Ended
Jun. 30, 2011
Marketable Securities [Abstract]  
Investments in marketable securities
                 
    Amortized     Fair  
(millions)   Cost     Value  
 
Corporate debt securities
  $ 167     $ 167  
U.S. government and agency debt securities
    63       63  
Asset-backed debt securities
    18       18  
Certificates of deposit
    49       49  
Municipal debt securities
    26       26  
 
Total marketable securities
  $ 323     $ 323  
 
Contractual maturities of marketable securities
                 
    Amortized     Fair  
(millions)   Cost     Value  
 
Due in 1 year or less
  $ 158     $ 158  
Due in 1-5 years
    165       165  
Due in more than 5 years
           
 
Total marketable securities
  $ 323     $ 323