Cash Equivalents and Investments - Unrealized Loss for Trading and Available for Sale Investments (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 1,421.6	$ 1,067.2
Unrealized loss, less than 12 months	(4.2)	(1.7)
Fair value, 12 months or greater	89.7	0.0
Unrealized loss, 12 months or greater	(0.4)	0.0
Total fair value, Available-for-sale investments in continuous unrealized loss position	1,511.3	1,067.2
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(4.6)	(1.7)
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	1,419.5	1,065.2
Unrealized loss, less than 12 months	(3.5)	(1.6)
Fair value, 12 months or greater	89.7	0.0
Unrealized loss, 12 months or greater	(0.4)	0.0
Total fair value, Available-for-sale investments in continuous unrealized loss position	1,509.2	1,065.2
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(3.9)	(1.6)
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	274.2	221.9
Unrealized loss, less than 12 months	(0.4)	(0.3)
Fair value, 12 months or greater	30.8	0.0
Unrealized loss, 12 months or greater	(0.1)	0.0
Total fair value, Available-for-sale investments in continuous unrealized loss position	305.0	221.9
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.5)	(0.3)
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	3.3
Unrealized loss, less than 12 months	0.0
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	3.3
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	0.0
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	687.9	515.9
Unrealized loss, less than 12 months	(2.3)	(1.1)
Fair value, 12 months or greater	58.9	0.0
Unrealized loss, 12 months or greater	(0.3)	0.0
Total fair value, Available-for-sale investments in continuous unrealized loss position	746.8	515.9
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(2.6)	(1.1)
Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	9.5	24.6
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	9.5	24.6
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	185.3	113.8
Unrealized loss, less than 12 months	(0.4)	(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	185.3	113.8
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.4)	(0.1)
US government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	259.3	189.0
Unrealized loss, less than 12 months	(0.4)	(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	259.3	189.0
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.4)	(0.1)
Publicly-traded equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	2.1	2.0
Unrealized loss, less than 12 months	(0.7)	(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	2.1	2.0
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	$ (0.7)	$ (0.1)