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Cash Equivalents and Investments - Available for Sale Securities (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Available-for-sale securities:    
Available-for-sale securities, amortized cost $ 2,460.9 $ 2,745.7
Available-for-sale securities, gross unrealized gains 2.7 3.1
Available-for-sale securities, gross unrealized losses (0.7) (0.3)
Available-for-sale securities, estimated fair value 2,462.9 2,748.5
Trading securities:    
Trading securities, amortized cost 13.8 [1] 12.6 [1]
Trading securities, gross unrealized gains 0 [1] 0 [1]
Trading securities, gross unrealized losses 0 [1] 0 [1]
Trading securities, estimated fair value 13.8 [1] 12.6 [1]
Available-for-sale and Trading Investments [Abstract]    
Total investments, amortized cost 2,474.7 2,758.3
Total investments, gross unrealized gains 2.7 3.1
Total investments, gross unrealized loss (0.7) (0.3)
Total investments, estimated fair value 2,476.7 2,761.1
Cash Equivalents [Member]
   
Available-for-sale and Trading Investments [Abstract]    
Total investments, amortized cost 755.9 1,225.9
Total investments, gross unrealized gains 0 0
Total investments, gross unrealized loss 0 0
Total investments, estimated fair value 755.9 1,225.9
Restricted Investments [Member]
   
Available-for-sale and Trading Investments [Abstract]    
Total investments, amortized cost 92.4 105.5
Total investments, gross unrealized gains 0.1 0.1
Total investments, gross unrealized loss 0 0
Total investments, estimated fair value 92.5 105.6
Short-term investments [Member]
   
Available-for-sale and Trading Investments [Abstract]    
Total investments, amortized cost 613.0 441.3
Total investments, gross unrealized gains 0.4 0.3
Total investments, gross unrealized loss (0.3) (0.1)
Total investments, estimated fair value 613.1 441.5
Long-term investments [Member]
   
Available-for-sale and Trading Investments [Abstract]    
Total investments, amortized cost 1,013.4 985.6
Total investments, gross unrealized gains 2.2 2.7
Total investments, gross unrealized loss (0.4) (0.2)
Total investments, estimated fair value 1,015.2 988.1
Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 1,609.7 1,411.3
Available-for-sale securities, gross unrealized gains 2.6 3.0
Available-for-sale securities, gross unrealized losses (0.4) (0.2)
Available-for-sale securities, estimated fair value 1,611.9 1,414.1
Asset-backed Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 234.7 226.2
Available-for-sale securities, gross unrealized gains 0.2 0.3
Available-for-sale securities, gross unrealized losses (0.1) (0.1)
Available-for-sale securities, estimated fair value 234.8 226.4
Certificate of Deposit [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 44.7 41.9
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 44.7 41.9
Commercial Paper [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost   11.6
Available-for-sale securities, gross unrealized gains   0
Available-for-sale securities, gross unrealized losses   0
Available-for-sale securities, estimated fair value   11.6
Corporate Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 614.0 533.4
Available-for-sale securities, gross unrealized gains 2.0 2.3
Available-for-sale securities, gross unrealized losses (0.3) (0.1)
Available-for-sale securities, estimated fair value 615.7 535.6
Foreign Government Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 15.8 5.0
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 15.8 5.0
Government-Sponsored Enterprise Obligations [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 270.4 264.6
Available-for-sale securities, gross unrealized gains 0.2 0.3
Available-for-sale securities, gross unrealized losses 0 0
Available-for-sale securities, estimated fair value 270.6 264.9
US Government Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 430.1 328.6
Available-for-sale securities, gross unrealized gains 0.2 0.1
Available-for-sale securities, gross unrealized losses 0 0
Available-for-sale securities, estimated fair value 430.3 328.7
Publicly-traded Equity Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 2.9 3.0
Available-for-sale securities, gross unrealized gains 0 0
Available-for-sale securities, gross unrealized losses (0.3) (0.1)
Available-for-sale securities, estimated fair value 2.6 2.9
Money Market Funds [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 89.5 102.6
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 89.5 102.6
Mutual Funds [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 2.9 2.9
Available-for-sale securities, gross unrealized gains 0.1 0.1
Available-for-sale securities, gross unrealized losses 0 0
Available-for-sale securities, estimated fair value 3.0 3.0
Restricted Investments [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 92.4 105.5
Available-for-sale securities, gross unrealized gains 0.1 0.1
Available-for-sale securities, gross unrealized losses 0 0
Available-for-sale securities, estimated fair value 92.5 105.6
Certificate of Deposit [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 0.3 0.6
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 0.3 0.6
Commercial Paper [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost   10.8
Available-for-sale securities, gross unrealized gains   0
Available-for-sale securities, gross unrealized losses   0
Available-for-sale securities, estimated fair value   10.8
US Government-sponsored Enterprises Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost   6.1
Available-for-sale securities, gross unrealized gains   0
Available-for-sale securities, gross unrealized losses   0
Available-for-sale securities, estimated fair value   6.1
Money Market Funds [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 749.6 1,042.6
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 749.6 1,042.6
US Government Debt Securities [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 6.0 165.8
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 6.0 165.8
Cash Equivalents [Member]
   
Available-for-sale securities:    
Available-for-sale securities, amortized cost 755.9 1,225.9
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 $ 755.9 $ 1,225.9
[1] Balance includes the Company's non-qualified deferred compensation plan assets.