Cash Equivalents and Investments - Unrealized Loss for Trading and Available for Sale Investments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 1,067.2	$ 568.7
Unrealized loss, less than 12 months	(1.7)	(2.5)
Fair value, 12 months or greater	0	10.3
Unrealized loss, 12 months or greater	0	0
Total fair value, Available-for-sale investments in continuous unrealized loss position	1,067.2	579.0
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(1.7)	(2.5)
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	1,065.2	561.9
Unrealized loss, less than 12 months	(1.6)	(0.6)
Fair value, 12 months or greater	0	10.3
Unrealized loss, 12 months or greater	0	0
Total fair value, Available-for-sale investments in continuous unrealized loss position	1,065.2	572.2
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(1.6)	(0.6)
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	221.9	153.0
Unrealized loss, less than 12 months	(0.3)	(0.1)
Fair value, 12 months or greater	0	0.6
Unrealized loss, 12 months or greater	0	0
Total fair value, Available-for-sale investments in continuous unrealized loss position	221.9	153.6
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.3)	(0.1)
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	515.9	156.1
Unrealized loss, less than 12 months	(1.1)	(0.3)
Fair value, 12 months or greater	0	9.7
Unrealized loss, 12 months or greater	0	0
Total fair value, Available-for-sale investments in continuous unrealized loss position	515.9	165.8
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(1.1)	(0.3)
Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	24.6	10.0
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	24.6	10.0
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	0	0
Government-sponsored enterprise obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	113.8	123.1
Unrealized loss, less than 12 months	(0.1)	(0.1)
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	113.8	123.1
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.1)	(0.1)
US government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	189.0	119.7
Unrealized loss, less than 12 months	(0.1)	(0.1)
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	189.0	119.7
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.1)	(0.1)
Publicly-traded equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	2.0	6.8
Unrealized loss, less than 12 months	(0.1)	(1.9)
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	2.0	6.8
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	$ (0.1)	$ (1.9)