Cash Equivalents and Investments - Unrealized Loss on Available-for-Sale Securities (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 297.8	$ 1,421.6
Unrealized loss, less than 12 months	(2.0)	(4.2)
Fair value, 12 months or greater	106.2	89.7
Unrealized loss, 12 months or greater	(0.3)	(0.4)
Total fair value, available-for-sale investments in continuous unrealized loss position	404.0	1,511.3
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(2.3)	(4.6)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	293.3	1,419.5
Unrealized loss, less than 12 months	(0.3)	(3.5)
Fair value, 12 months or greater	106.2	89.7
Unrealized loss, 12 months or greater	(0.3)	(0.4)
Total fair value, available-for-sale investments in continuous unrealized loss position	399.5	1,509.2
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.6)	(3.9)
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	82.4	274.2
Unrealized loss, less than 12 months	0.0	(0.4)
Fair value, 12 months or greater	12.4	30.8
Unrealized loss, 12 months or greater	0.0	(0.1)
Total fair value, available-for-sale investments in continuous unrealized loss position	94.8	305.0
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	0.0	(0.5)
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	3.3	3.3
Unrealized loss, less than 12 months	0.0	0.0
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	3.3	3.3
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	0.0	0.0
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	153.9	687.9
Unrealized loss, less than 12 months	(0.3)	(2.3)
Fair value, 12 months or greater	93.8	58.9
Unrealized loss, 12 months or greater	(0.3)	(0.3)
Total fair value, available-for-sale investments in continuous unrealized loss position	247.7	746.8
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.6)	(2.6)
Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	13.5	9.5
Unrealized loss, less than 12 months	0.0	0.0
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	13.5	9.5
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	0.0	0.0
Government-sponsored enterprise obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	23.0	185.3
Unrealized loss, less than 12 months	0.0	(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	23.0	185.3
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	0.0	(0.4)
US government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	17.2	259.3
Unrealized loss, less than 12 months	0.0	(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	17.2	259.3
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	0.0	(0.4)
Publicly-traded equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	4.5	2.1
Unrealized loss, less than 12 months	(1.7)	(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	4.5	2.1
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	$ (1.7)	$ (0.7)