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Financial Instruments (Fair Value Of Marketable Equity Securities Schedule) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Mar. 31, 2012
Financial Instruments    
Fair value $ 59 $ 119 [1]
Marketable Equity Securities
   
Financial Instruments    
Adjusted Cost 21 32
Gross Unrealized Gains 38 87
Gross Unrealized Losses 0 0
Fair value $ 59 $ 119
[1] Derived from audited consolidated financial statements.