Cash Equivalents and Investments - Unrealized Loss on Available-for-Sale Securities (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 1,095.9	$ 978.7
Unrealized loss, less than 12 months	(2.7)	(3.6)
Fair value, 12 months or greater	50.9	76.7
Unrealized loss, 12 months or greater	(0.1)	(0.1)
Total fair value, available-for-sale investments in continuous unrealized loss position	1,146.8	1,055.4
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(2.8)	(3.7)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	1,095.9	974.1
Unrealized loss, less than 12 months	(2.7)	(2.9)
Fair value, 12 months or greater	50.9	76.7
Unrealized loss, 12 months or greater	(0.1)	(0.1)
Total fair value, available-for-sale investments in continuous unrealized loss position	1,146.8	1,050.8
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(2.8)	(3.0)
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	260.9	122.2
Unrealized loss, less than 12 months	(0.4)	(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	260.9	122.2
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.4)	(0.2)
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	432.8	470.8
Unrealized loss, less than 12 months	(1.4)	(1.9)
Fair value, 12 months or greater	50.9	76.7
Unrealized loss, 12 months or greater	(0.1)	(0.1)
Total fair value, available-for-sale investments in continuous unrealized loss position	483.7	547.5
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(1.5)	(2.0)
Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	23.3	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	23.3	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	106.1	106.7
Unrealized loss, less than 12 months	(0.4)	(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	106.1	106.7
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.4)	(0.3)
US government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	272.8	254.1
Unrealized loss, less than 12 months	(0.4)	(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	272.8	254.1
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	$ (0.4)	(0.4)
Publicly-traded equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months		4.6
Unrealized loss, less than 12 months		(0.7)
Fair value, 12 months or greater		0.0
Unrealized loss, 12 months or greater		0.0
Total fair value, available-for-sale investments in continuous unrealized loss position		4.6
Total unrealized loss, available-for-sale investments in continuous unrealized loss position		$ (0.7)