Cash Equivalents and Investments - Unrealized Loss on Available-for-Sale Securities (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 885.3	$ 1,067.2
Unrealized loss, less than 12 months	(2.1)	(1.7)
Fair value, 12 months or greater	51.7	0.0
Unrealized loss, 12 months or greater	(0.1)	0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	937.0	1,067.2
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(2.2)	(1.7)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	885.3	1,065.2
Unrealized loss, less than 12 months	(2.1)	(1.6)
Fair value, 12 months or greater	51.7	0.0
Unrealized loss, 12 months or greater	(0.1)	0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	937.0	1,065.2
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(2.2)	(1.6)
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	188.5	221.9
Unrealized loss, less than 12 months	(0.2)	(0.3)
Fair value, 12 months or greater	34.2	0.0
Unrealized loss, 12 months or greater	0.0	0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	222.7	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	522.0	515.9
Unrealized loss, less than 12 months	(1.8)	(1.1)
Fair value, 12 months or greater	17.5	0.0
Unrealized loss, 12 months or greater	(0.1)	0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	539.5	515.9
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(1.9)	(1.1)
Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	13.5	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	13.5	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	41.8	113.8
Unrealized loss, less than 12 months	0.0	(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	41.8	113.8
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	0.0	(0.1)
US government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	119.5	189.0
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	119.5	189.0
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
Unrealized loss, less than 12 months		(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		2.0
Total unrealized loss, available-for-sale investments in continuous unrealized loss position		$ (0.1)