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Cash, Cash Equivalents and Investments - Available for Sale Securities (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Available-for-sale securities:    
Available-for-sale securities, amortized cost $ 1,372.1 $ 999.0
Available-for-sale securities, gross unrealized gains 1.4 3.1
Available-for-sale securities, gross unrealized losses (0.8) (0.5)
Available-for-sale securities, estimated fair value 1,372.7 1,001.6
Trading securities:    
Trading securities, amortized cost   8.1 [1]
Trading securities, gross unrealized gains   0 [1]
Trading securities, gross unrealized losses   0 [1]
Trading securities, estimated fair value   8.1 [1]
Available-for-sale and Trading Investments [Abstract]    
Total investments, amortized cost 1,381.4 1,007.1
Total investments, gross unrealized gains 1.4 3.1
Total investments, gross unrealized losses (0.8) (0.5)
Total investments, estimated fair value 1,382.0 1,009.7
Short-term investments [Member]
   
Available-for-sale and Trading Investments [Abstract]    
Total investments, amortized cost 640.9 473.6
Total investments, gross unrealized gains 0.4 0.9
Total investments, gross unrealized losses 0 0
Total investments, estimated fair value 641.3 474.5
Long-term investments [Member]
   
Available-for-sale and Trading Investments [Abstract]    
Total investments, amortized cost 740.5 533.5
Total investments, gross unrealized gains 1.0 2.2
Total investments, gross unrealized losses (0.8) (0.5)
Total investments, estimated fair value 740.7 535.2
Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 1,372.1 999.0
Available-for-sale securities, gross unrealized gains 1.4 3.1
Available-for-sale securities, gross unrealized losses (0.8) (0.5)
Available-for-sale securities, estimated fair value 1,372.7 1,001.6
US Government Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 301.1 158.2
Available-for-sale securities, gross unrealized gains 0 0.2
Available-for-sale securities, gross unrealized losses (0.1) 0
Available-for-sale securities, estimated fair value 301.0 158.4
US Government-sponsored Enterprises Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 406.3 213.8
Available-for-sale securities, gross unrealized gains 0.3 0.4
Available-for-sale securities, gross unrealized losses (0.1) (0.2)
Available-for-sale securities, estimated fair value 406.5 214.0
Foreign Government Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 0 46.8
Available-for-sale securities, gross unrealized gains 0 0.2
Available-for-sale securities, gross unrealized losses 0 0
Available-for-sale securities, estimated fair value 0 47.0
Certificates of Deposit [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 31.8 20.9
Available-for-sale securities, gross unrealized gains 0 0.1
Available-for-sale securities, gross unrealized losses 0 0
Available-for-sale securities, estimated fair value 31.8 21.0
Commercial Paper [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost   9.5
Available-for-sale securities, gross unrealized gains   0
Available-for-sale securities, gross unrealized losses   0
Available-for-sale securities, estimated fair value   9.5
Asset-backed Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 124.7 90.1
Available-for-sale securities, gross unrealized gains 0.1 0
Available-for-sale securities, gross unrealized losses (0.1) (0.1)
Available-for-sale securities, estimated fair value 124.7 90.0
Corporate Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 508.2 459.7
Available-for-sale securities, gross unrealized gains 1.0 2.2
Available-for-sale securities, gross unrealized losses (0.5) (0.2)
Available-for-sale securities, estimated fair value 508.7 461.7
Mutual Funds [Member]
   
Trading securities:    
Trading securities, amortized cost 9.3 [1]  
Trading securities, gross unrealized gains 0 [1]  
Trading securities, gross unrealized losses 0 [1]  
Trading securities, estimated fair value $ 9.3 [1]  
[1] Balance includes the Company's non-qualified deferred compensation plan assets. For additional information, see Note 12, Employee Benefits Plans, under the section Deferred Compensation Plan.