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Financial Instruments (Fair Value Of Short-Term Investments Schedule) (Details) (USD $)
In Millions
Sep. 30, 2011
Mar. 31, 2011
Financial Instruments [Line Items]  
Short-term investments$ 355$ 497[1]
Commercial Paper [Member] | Short-Term Investments [Member]
  
Financial Instruments [Line Items]  
Cost or Amortized Cost1718
Gross Unrealized Gains  
Gross Unrealized Losses  
Short-term investments1718
U.S. Treasury Securities [Member] | Short-Term Investments [Member]
  
Financial Instruments [Line Items]  
Cost or Amortized Cost82124
Gross Unrealized Gains  
Gross Unrealized Losses  
Short-term investments82124
U.S. Agency Securities [Member] | Short-Term Investments [Member]
  
Financial Instruments [Line Items]  
Cost or Amortized Cost86102
Gross Unrealized Gains  
Gross Unrealized Losses  
Short-term investments86102
Corporate Bonds [Member] | Short-Term Investments [Member]
  
Financial Instruments [Line Items]  
Cost or Amortized Cost169252
Gross Unrealized Gains11
Gross Unrealized Losses  
Short-term investments170253
Short-Term Investments [Member]
  
Financial Instruments [Line Items]  
Cost or Amortized Cost354496
Gross Unrealized Gains11
Gross Unrealized Losses  
Short-term investments$ 355$ 497
[1]Derived from audited consolidated financial statements.