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Financial Instruments (Details 2) (USD $)
In Millions
Sep. 30, 2011
Mar. 31, 2011
Amounts included in accumulated other comprehensive income from available-for-sale securities (pre-tax):  
Unrealized losses$ 3.8 
Cash and Cash Equivalents [Member]
  
Measured at fair value:  
Total debt and equity investments measured at fair value61.5563.4
Total cash, cash equivalents and investments1,459.71,660.9
Amounts included in accumulated other comprehensive income from available-for-sale securities (pre-tax):  
Unrealized losses0[1]0[1]
Cash and Cash Equivalents [Member] | U.S. Treasury securities [Member]
  
Measured at fair value:  
Available-for-sale0525.0
Cash and Cash Equivalents [Member] | Certificates of deposit [Member]
  
Measured at fair value:  
Available-for-sale61.538.4
Cash and Cash Equivalents [Member] | Mutual funds [Member]
  
Measured at fair value:  
Trading00
Cash and Cash Equivalents [Member] | Auction Rate Securities [Member]
  
Measured at fair value:  
Available-for-sale00
Cash and Cash Equivalents [Member] | Cash on hand [Member]
  
Measured at fair value:  
Other measurement basis895.1412.8
Cash and Cash Equivalents [Member] | Money-market funds [Member]
  
Measured at fair value:  
Other measurement basis503.1684.7
Short-term Investments [Member]
  
Measured at fair value:  
Total debt and equity investments measured at fair value30.927.8
Total cash, cash equivalents and investments30.927.8
Amounts included in accumulated other comprehensive income from available-for-sale securities (pre-tax):  
Unrealized losses0[1]0[1]
Short-term Investments [Member] | U.S. Treasury securities [Member]
  
Measured at fair value:  
Available-for-sale30.927.8
Short-term Investments [Member] | Certificates of deposit [Member]
  
Measured at fair value:  
Available-for-sale00
Short-term Investments [Member] | Mutual funds [Member]
  
Measured at fair value:  
Trading00
Short-term Investments [Member] | Auction Rate Securities [Member]
  
Measured at fair value:  
Available-for-sale00
Short-term Investments [Member] | Cash on hand [Member]
  
Measured at fair value:  
Other measurement basis00
Short-term Investments [Member] | Money-market funds [Member]
  
Measured at fair value:  
Other measurement basis00
Long- term Investments [Member]
  
Measured at fair value:  
Total debt and equity investments measured at fair value61.967.8
Total cash, cash equivalents and investments61.967.8
Amounts included in accumulated other comprehensive income from available-for-sale securities (pre-tax):  
Unrealized losses3.8[1]2.6[1]
Long- term Investments [Member] | U.S. Treasury securities [Member]
  
Measured at fair value:  
Available-for-sale19.122.1
Long- term Investments [Member] | Certificates of deposit [Member]
  
Measured at fair value:  
Available-for-sale00
Long- term Investments [Member] | Mutual funds [Member]
  
Measured at fair value:  
Trading17.218.5
Long- term Investments [Member] | Auction Rate Securities [Member]
  
Measured at fair value:  
Available-for-sale25.627.2
Long- term Investments [Member] | Cash on hand [Member]
  
Measured at fair value:  
Other measurement basis00
Long- term Investments [Member] | Money-market funds [Member]
  
Measured at fair value:  
Other measurement basis00
Auction Rate Securities [Member]
  
Measured at fair value:  
Available-for-sale$ 25.6$ 27.2
[1]The unrealized losses on available-for-sale securities at June 30, 2011 and March 31, 2011 relate to the auction rate securities.