Cash Equivalents and Investments - Unrealized Loss on Available-for-Sale Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 750.9	$ 1,067.2
Unrealized loss, less than 12 months	(0.7)	(1.7)
Fair value, 12 months or greater	1.7	0
Unrealized loss, 12 months or greater	0	0
Total fair value, available-for-sale investments in continuous unrealized loss position	752.6	1,067.2
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.7)	(1.7)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	750.9	1,065.2
Unrealized loss, less than 12 months	(0.7)	(1.6)
Fair value, 12 months or greater	1.7	0
Unrealized loss, 12 months or greater	0	0
Total fair value, available-for-sale investments in continuous unrealized loss position	752.6	1,065.2
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.7)	(1.6)
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	222.4	221.9
Unrealized loss, less than 12 months	(0.2)	(0.3)
Fair value, 12 months or greater	1.7	0
Unrealized loss, 12 months or greater	0	0
Total fair value, available-for-sale investments in continuous unrealized loss position	224.1	221.9
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.2)	(0.3)
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	391.5	515.9
Unrealized loss, less than 12 months	(0.5)	(1.1)
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	391.5	515.9
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.5)	(1.1)
Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	29.1	24.6
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	29.1	24.6
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	60.4	113.8
Unrealized loss, less than 12 months	0	(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	60.4	113.8
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	0	(0.1)
US government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	47.5	189.0
Unrealized loss, less than 12 months	0	(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	47.5	189.0
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	0	(0.1)
Publicly-traded equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months		2.0
Unrealized loss, less than 12 months		(0.1)
Fair value, 12 months or greater		0
Unrealized loss, 12 months or greater		0
Total fair value, available-for-sale investments in continuous unrealized loss position		2.0
Total unrealized loss, available-for-sale investments in continuous unrealized loss position		$ (0.1)