Cash Equivalents and Investments - Unrealized Loss on Available-for-Sale Securities (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 1,083.1	$ 978.7
Unrealized loss, less than 12 months	(3.3)	(3.6)
Fair value, 12 months or greater	0.0	76.7
Unrealized loss, 12 months or greater	0.0	(0.1)
Total fair value, available-for-sale investments in continuous unrealized loss position	1,083.1	1,055.4
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(3.3)	(3.7)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	1,077.8	974.1
Unrealized loss, less than 12 months	(2.4)	(2.9)
Fair value, 12 months or greater	0.0	76.7
Unrealized loss, 12 months or greater	0.0	(0.1)
Total fair value, available-for-sale investments in continuous unrealized loss position	1,077.8	1,050.8
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(2.4)	(3.0)
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	270.1	122.2
Unrealized loss, less than 12 months	(0.3)	(0.2)
Fair value, 12 months or greater	0.0	0.0
Unrealized loss, 12 months or greater	0.0	0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	270.1	122.2
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.3)	(0.2)
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	391.3	470.8
Unrealized loss, less than 12 months	(1.0)	(1.9)
Fair value, 12 months or greater	0.0	76.7
Unrealized loss, 12 months or greater	0.0	(0.1)
Total fair value, available-for-sale investments in continuous unrealized loss position	391.3	547.5
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(1.0)	(2.0)
Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	34.2	20.3
Unrealized loss, less than 12 months	(0.1)	(0.1)
Fair value, 12 months or greater	0.0	0.0
Unrealized loss, 12 months or greater	0.0	0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	34.2	20.3
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.1)	(0.1)
Government-sponsored enterprise obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	132.5	106.7
Unrealized loss, less than 12 months	(0.5)	(0.3)
Fair value, 12 months or greater	0.0	0.0
Unrealized loss, 12 months or greater	0.0	0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	132.5	106.7
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.5)	(0.3)
US government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	249.7	254.1
Unrealized loss, less than 12 months	(0.5)	(0.4)
Fair value, 12 months or greater	0.0	0.0
Unrealized loss, 12 months or greater	0.0	0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	249.7	254.1
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.5)	(0.4)
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	5.3	4.6
Unrealized loss, less than 12 months	(0.9)	(0.7)
Fair value, 12 months or greater	0.0	0.0
Unrealized loss, 12 months or greater	0.0	0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	5.3	4.6
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	$ (0.9)	$ (0.7)