Cash Equivalents and Investments - Available for Sale Securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale securities:
Available-for-sale securities, amortized cost	$ 3,356.1	$ 2,793.0
Available-for-sale Securities, gross unrealized gain	37.9	27.1
Available-for-sale securities, gross unrealized loss	(5.4)	(3.7)
Available-for-sale securities:	3,388.6	2,816.4
Cash equivalents
Available-for-sale securities:
Available-for-sale securities, amortized cost	1,279.0	907.1
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities:	1,279.0	907.1
Restricted investments
Available-for-sale securities:
Available-for-sale securities, amortized cost	41.8	42.9
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities:	41.8	42.9
Short-term investments
Available-for-sale securities:
Available-for-sale securities, amortized cost	1,027.2	753.4
Available-for-sale Securities, gross unrealized gain	0.1	0.1
Available-for-sale securities, gross unrealized loss	(1.2)	(1.2)
Available-for-sale securities:	1,026.1	752.3
Long-term investments
Available-for-sale securities:
Available-for-sale securities, amortized cost	992.2	1,073.7
Available-for-sale Securities, gross unrealized gain	0.4	0.6
Available-for-sale securities, gross unrealized loss	(4.2)	(2.5)
Available-for-sale securities:	988.4	1,071.8
Prepaid expenses and other current assets
Available-for-sale securities:
Available-for-sale securities:	31.4	4.0
Other Long-Term Assets
Available-for-sale securities:
Available-for-sale securities, amortized cost	15.9	15.9
Available-for-sale Securities, gross unrealized gain	37.4	26.4
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities:	53.3	42.3
Fixed Income Securities
Available-for-sale securities:
Available-for-sale securities, amortized cost	2,370.4	2,179.6
Available-for-sale Securities, gross unrealized gain	0.5	0.7
Available-for-sale securities, gross unrealized loss	(5.4)	(3.0)
Available-for-sale securities:	2,365.5	2,177.3
Asset-backed securities
Available-for-sale securities:
Available-for-sale securities, amortized cost	287.1	303.0
Available-for-sale Securities, gross unrealized gain	0.0	0.2
Available-for-sale securities, gross unrealized loss	(0.6)	(0.2)
Available-for-sale securities:	286.5	303.0
Commercial paper
Available-for-sale securities:
Available-for-sale securities, amortized cost	83.8	66.1
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities:	83.8	66.1
Commercial paper
Available-for-sale securities:
Available-for-sale securities, amortized cost	217.1	147.7
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities:	217.1	147.7
Corporate debt securities
Available-for-sale securities:
Available-for-sale securities, amortized cost	929.6	846.5
Available-for-sale Securities, gross unrealized gain	0.4	0.4
Available-for-sale securities, gross unrealized loss	(3.0)	(2.0)
Available-for-sale securities:	927.0	844.9
Foreign government debt securities
Available-for-sale securities:
Available-for-sale securities, amortized cost	62.9	34.0
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	(0.2)	(0.1)
Available-for-sale securities:	62.7	33.9
Time deposits
Available-for-sale securities:
Available-for-sale securities, amortized cost	239.2	264.6
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities:	239.2	264.6
U.S. government agency securities
Available-for-sale securities:
Available-for-sale securities, amortized cost	143.9	127.0
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	(0.7)	(0.3)
Available-for-sale securities:	143.2	126.7
U.S. government securities
Available-for-sale securities:
Available-for-sale securities, amortized cost	406.8	390.7
Available-for-sale Securities, gross unrealized gain	0.1	0.1
Available-for-sale securities, gross unrealized loss	(0.9)	(0.4)
Available-for-sale securities:	406.0	390.4
Money market funds
Available-for-sale securities:
Available-for-sale securities, amortized cost	969.8	592.2
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities:	969.8	592.2
Privately-held debt and redeemable preferred stock securities
Available-for-sale securities:
Available-for-sale securities, amortized cost	15.9	15.9
Available-for-sale Securities, gross unrealized gain	37.4	26.4
Available-for-sale securities, gross unrealized loss	0.0	0.0
Available-for-sale securities:	53.3	42.3
Publicly-traded equity securities
Available-for-sale securities:
Available-for-sale securities, amortized cost	0.0	5.3
Available-for-sale Securities, gross unrealized gain	0.0	0.0
Available-for-sale securities, gross unrealized loss	0.0	(0.7)
Available-for-sale securities:	$ 0.0	$ 4.6