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Marketable Securities (Tables)
12 Months Ended
Mar. 31, 2012
Marketable Securities [Abstract]  
Available-for-sale Securities
                 
    AT MARCH 31, 2011  
(in millions)   AGGREGATE
COST BASIS
    AGGREGATE
FAIR VALUE
 

U.S. Treasury and agency securities

  $ 60     $ 60  

Municipal securities

    2       2  

Corporate debt securities

    117       117  
    $ 179     $ 179