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Cash, Cash Equivalents and Investments - Available for Sale Securities (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Available-for-sale securities:    
Available-for-sale securities, amortized cost $ 2,634.1 $ 2,722.8
Available-for-sale securities, gross unrealized gains 3.7 1.4
Available-for-sale securities, gross unrealized losses (0.4) (0.8)
Available-for-sale securities, estimated fair value 2,637.4 2,723.4
Trading securities:    
Trading securities, amortized cost 11.9 [1] 9.3 [1]
Trading securities, gross unrealized gains 0 [1] 0 [1]
Trading securities, gross unrealized losses 0 [1] 0 [1]
Trading securities, estimated fair value 11.9 [1] 9.3 [1]
Available-for-sale and Trading Investments [Abstract]    
Total investments, amortized cost 2,646.0 2,732.1
Total investments, gross unrealized gains 3.7 1.4
Total investments, gross unrealized losses (0.4) (0.8)
Total investments, estimated fair value 2,649.3 2,732.7
Cash Equivalents [Member]
   
Available-for-sale and Trading Investments [Abstract]    
Total investments, amortized cost 1,346.6 1,350.7
Total investments, gross unrealized gains 0 0
Total investments, gross unrealized losses 0 0
Total investments, estimated fair value 1,346.6 1,350.7
Short-term investments [Member]
   
Available-for-sale and Trading Investments [Abstract]    
Total investments, amortized cost 515.8 640.9
Total investments, gross unrealized gains 1.6 0.4
Total investments, gross unrealized losses 0 0
Total investments, estimated fair value 517.4 641.3
Long-term investments [Member]
   
Available-for-sale and Trading Investments [Abstract]    
Total investments, amortized cost 783.6 740.5
Total investments, gross unrealized gains 2.1 1.0
Total investments, gross unrealized losses (0.4) (0.8)
Total investments, estimated fair value 785.3 740.7
Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 2,631.5 2,722.8
Available-for-sale securities, gross unrealized gains 2.4 1.4
Available-for-sale securities, gross unrealized losses (0.4) (0.8)
Available-for-sale securities, estimated fair value 2,633.5 2,723.4
US Government Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 376.9 301.1
Available-for-sale securities, gross unrealized gains 0 0
Available-for-sale securities, gross unrealized losses (0.2) (0.1)
Available-for-sale securities, estimated fair value 376.7 301.0
US Government-sponsored Enterprises Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 275.7 430.8
Available-for-sale securities, gross unrealized gains 0.5 0.3
Available-for-sale securities, gross unrealized losses 0 (0.1)
Available-for-sale securities, estimated fair value 276.2 431.0
Certificates of Deposit [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 25.6 31.8
Available-for-sale securities, gross unrealized gains 0 0
Available-for-sale securities, gross unrealized losses 0 0
Available-for-sale securities, estimated fair value 25.6 31.8
Commercial Paper [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 15.0 10.0
Available-for-sale securities, gross unrealized gains 0 0
Available-for-sale securities, gross unrealized losses 0 0
Available-for-sale securities, estimated fair value 15.0 10.0
Asset-backed Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 141.8 124.7
Available-for-sale securities, gross unrealized gains 0.2 0.1
Available-for-sale securities, gross unrealized losses 0 (0.1)
Available-for-sale securities, estimated fair value 142.0 124.7
Corporate Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 566.9 508.2
Available-for-sale securities, gross unrealized gains 1.7 1.0
Available-for-sale securities, gross unrealized losses (0.2) (0.5)
Available-for-sale securities, estimated fair value 568.4 508.7
Money Market Funds [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 1,229.6 1,316.2
Available-for-sale securities, gross unrealized gains 0 0
Available-for-sale securities, gross unrealized losses 0 0
Available-for-sale securities, estimated fair value 1,229.6 1,316.2
Equity Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 2.6  
Available-for-sale securities, gross unrealized gains 1.3  
Available-for-sale securities, gross unrealized losses 0  
Available-for-sale securities, estimated fair value $ 3.9  
[1] Balance includes the Company's non-qualified deferred compensation plan assets. For additional information, see Note 12, Employee Benefit Plans, under the section Deferred Compensation Plan.