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Financial Instruments (Components Of Cash, Cash Equivalents And Investments) (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Mar. 31, 2012
Auction Rate Securities [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale $ 19.9 $ 26.9
Cash, Cash Equivalents And Investments [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total debt and equity investments measured at fair value 75.0 49.9
Total cash, cash equivalents and investments 1,379.2 1,496.9
Cash, Cash Equivalents And Investments [Member] | United States Treasury Securities [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale 75.0 49.9
Cash, Cash Equivalents And Investments [Member] | Cash On Hand [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Other measurement basis 1,281.8 632.6
Cash, Cash Equivalents And Investments [Member] | Certificates Of Deposit [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Other measurement basis 22.2 45.1
Cash, Cash Equivalents And Investments [Member] | Money-Market Funds [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Other measurement basis 0.2 769.3
Short-Term Investments [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total debt and equity investments measured at fair value 131.2 86.1
Total cash, cash equivalents and investments 131.2 86.1
Short-Term Investments [Member] | United States Treasury Securities [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale 102.1 86.1
Short-Term Investments [Member] | United States Government Agency Bonds [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale 5.0  
Short-Term Investments [Member] | Corporate Bonds And Commercial Paper [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale 24.1  
Long-Term Investments [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total debt and equity investments measured at fair value 71.5 52.6
Total cash, cash equivalents and investments 71.5 52.6
Amounts included in accumulated other comprehensive income from available-for-sale securities (pre-tax):    
Unrealized losses 1.7 [1] 2.4 [1]
Long-Term Investments [Member] | United States Treasury Securities [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale   6.1
Long-Term Investments [Member] | Auction Rate Securities [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale 19.9 26.9
Long-Term Investments [Member] | United States Government Agency Bonds [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale 18.4  
Long-Term Investments [Member] | Corporate Bonds And Commercial Paper [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Available-for-sale 12.3  
Long-Term Investments [Member] | Mutual Funds [Member]
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Trading $ 20.9 $ 19.6
[1] The unrealized losses on available-for-sale securities at March 31, 2013 and 2012 relate to the auction rate securities.