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Cash Equivalents and Investments - Available-For-Sale and Trading Securities (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Available-for-sale securities:    
Available-for-sale securities, amortized cost $ 3,464.6 $ 2,785.1
Available-for-sale securities, gross unrealized gains 1.1 0.7
Available-for-sale securities, gross unrealized losses (3.3) (3.7)
Available-for-sale securities, estimated fair value 3,462.4 2,782.1
Available-for-sale and Trading Securities [Abstract]    
Total investments, amortized cost 3,488.5 2,806.1
Total investments, gross unrealized gains 1.1 0.7
Total investments, gross unrealized losses (3.3) (3.7)
Total investments, estimated fair value 3,486.3 2,803.1
Debt Securities [Member]    
Available-for-sale securities:    
Available-for-sale securities, amortized cost 2,243.5 2,179.6
Available-for-sale securities, gross unrealized gains 1.1 0.7
Available-for-sale securities, gross unrealized losses (2.4) (3.0)
Available-for-sale securities, estimated fair value 2,242.2 2,177.3
Asset-backed securities [Member]    
Available-for-sale securities:    
Available-for-sale securities, amortized cost 309.6 303.0
Available-for-sale securities, gross unrealized gains 0.0 0.2
Available-for-sale securities, gross unrealized losses (0.3) (0.2)
Available-for-sale securities, estimated fair value 309.3 303.0
Certificates of deposit [Member]    
Available-for-sale securities:    
Available-for-sale securities, amortized cost 45.7 66.1
Available-for-sale securities, gross unrealized gains 0.0 0.0
Available-for-sale securities, gross unrealized losses 0.0 0.0
Available-for-sale securities, estimated fair value 45.7 66.1
Commercial paper [Member]    
Available-for-sale securities:    
Available-for-sale securities, amortized cost 167.8 147.7
Available-for-sale securities, gross unrealized gains 0.0 0.0
Available-for-sale securities, gross unrealized losses 0.0 0.0
Available-for-sale securities, estimated fair value 167.8 147.7
Corporate debt securities [Member]    
Available-for-sale securities:    
Available-for-sale securities, amortized cost 863.1 846.5
Available-for-sale securities, gross unrealized gains 1.0 0.4
Available-for-sale securities, gross unrealized losses (1.0) (2.0)
Available-for-sale securities, estimated fair value 863.1 844.9
Foreign government debt securities [Member]    
Available-for-sale securities:    
Available-for-sale securities, amortized cost 44.0 34.0
Available-for-sale securities, gross unrealized gains 0.0 0.0
Available-for-sale securities, gross unrealized losses (0.1) (0.1)
Available-for-sale securities, estimated fair value 43.9 33.9
Time deposits [Member]    
Available-for-sale securities:    
Available-for-sale securities, amortized cost 290.7 264.6
Available-for-sale securities, gross unrealized gains 0.0 0.0
Available-for-sale securities, gross unrealized losses 0.0 0.0
Available-for-sale securities, estimated fair value 290.7 264.6
Government-sponsored enterprise obligations [Member]    
Available-for-sale securities:    
Available-for-sale securities, amortized cost 163.5 127.0
Available-for-sale securities, gross unrealized gains 0.0 0.0
Available-for-sale securities, gross unrealized losses (0.5) (0.3)
Available-for-sale securities, estimated fair value 163.0 126.7
Mutual Fund [Member]    
Available-for-sale securities:    
Available-for-sale securities, amortized cost 8.3 8.0
Available-for-sale securities, gross unrealized gains 0.0 0.0
Available-for-sale securities, gross unrealized losses 0.0 0.0
Available-for-sale securities, estimated fair value 8.3 8.0
Trading securities:    
Trading securities, amortized cost 23.9 21.0
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Trading securities, estimated fair value 23.9 21.0
US government securities [Member]    
Available-for-sale securities:    
Available-for-sale securities, amortized cost 359.1 390.7
Available-for-sale securities, gross unrealized gains 0.1 0.1
Available-for-sale securities, gross unrealized losses (0.5) (0.4)
Available-for-sale securities, estimated fair value 358.7 390.4
Money market funds [Member]    
Available-for-sale securities:    
Available-for-sale securities, amortized cost 1,206.6 592.2
Available-for-sale securities, gross unrealized gains 0.0 0.0
Available-for-sale securities, gross unrealized losses 0.0 0.0
Available-for-sale securities, estimated fair value 1,206.6 592.2
Equity Securities [Member]    
Available-for-sale securities:    
Available-for-sale securities, amortized cost 6.2 5.3
Available-for-sale securities, gross unrealized gains 0.0 0.0
Available-for-sale securities, gross unrealized losses (0.9) (0.7)
Available-for-sale securities, estimated fair value 5.3 4.6
Cash equivalents [Member]    
Available-for-sale and Trading Securities [Abstract]    
Total investments, amortized cost 1,589.2 907.1
Total investments, gross unrealized gains 0.0 0.0
Total investments, gross unrealized losses 0.0 0.0
Total investments, estimated fair value 1,589.2 907.1
Restricted investments [Member]    
Available-for-sale and Trading Securities [Abstract]    
Total investments, amortized cost 75.0 71.9
Total investments, gross unrealized gains 0.0 0.0
Total investments, gross unrealized losses 0.0 0.0
Total investments, estimated fair value 75.0 71.9
Short-term investments [Member]    
Available-for-sale and Trading Securities [Abstract]    
Total investments, amortized cost 754.0 753.4
Total investments, gross unrealized gains 0.1 0.1
Total investments, gross unrealized losses (1.4) (1.2)
Total investments, estimated fair value 752.7 752.3
Long-term investments [Member]    
Available-for-sale and Trading Securities [Abstract]    
Total investments, amortized cost 1,070.3 1,073.7
Total investments, gross unrealized gains 1.0 0.6
Total investments, gross unrealized losses (1.9) (2.5)
Total investments, estimated fair value $ 1,069.4 $ 1,071.8