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Financial Instruments (Fair Value Of Short-Term Investments) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2011
Mar. 31, 2011
Financial Instruments [Line Items]    
Fair Value $ 406 $ 497 [1]
Short-Term Investments [Member]
   
Financial Instruments [Line Items]    
Cost or Amortized Cost 405 496
Gross Unrealized Gains 1 1
Gross Unrealized Losses     
Fair Value 406 497
Commercial Paper [Member] | Short-Term Investments [Member]
   
Financial Instruments [Line Items]    
Cost or Amortized Cost 12 18
Gross Unrealized Losses     
Fair Value 12 18
U.S. Treasury Securities [Member] | Short-Term Investments [Member]
   
Financial Instruments [Line Items]    
Cost or Amortized Cost 123 124
Gross Unrealized Losses     
Fair Value 123 124
U.S. Agency Securities [Member] | Short-Term Investments [Member]
   
Financial Instruments [Line Items]    
Cost or Amortized Cost 120 102
Gross Unrealized Losses     
Fair Value 120 102
Corporate Bonds [Member] | Short-Term Investments [Member]
   
Financial Instruments [Line Items]    
Cost or Amortized Cost 150 252
Gross Unrealized Gains 1 1
Gross Unrealized Losses     
Fair Value $ 151 $ 253
[1] Derived from audited consolidated financial statements.