Cash Equivalents and Investments - Unrealized Loss on Available-for-Sale Securities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Unrealized gain (loss) on privately-held debt securities	$ 0.0	$ 10.0	$ 0.0	$ 10.0
Fair value, less than 12 months	854.9		854.9		$ 1,067.2
Unrealized loss, less than 12 months	(1.0)		(1.0)		(1.7)
Fair value, 12 months or greater	20.8		20.8		0.0
Unrealized loss, 12 months or greater	0.0		0.0		0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	875.7		875.7		1,067.2
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(1.0)		(1.0)		(1.7)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	854.9		854.9		1,065.2
Unrealized loss, less than 12 months	(1.0)		(1.0)		(1.6)
Fair value, 12 months or greater	20.8		20.8		0.0
Unrealized loss, 12 months or greater	0.0		0.0		0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	875.7		875.7		1,065.2
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(1.0)		(1.0)		(1.6)
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	190.3		190.3		221.9
Unrealized loss, less than 12 months	(0.1)		(0.1)		(0.3)
Fair value, 12 months or greater	10.2		10.2		0.0
Unrealized loss, 12 months or greater	0.0		0.0		0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	200.5		200.5		221.9
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.1)		(0.1)		(0.3)
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	519.9		519.9		515.9
Unrealized loss, less than 12 months	(0.9)		(0.9)		(1.1)
Fair value, 12 months or greater	10.6		10.6		0.0
Unrealized loss, 12 months or greater	0.0		0.0		0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	530.5		530.5		515.9
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	(0.9)		(0.9)		(1.1)
Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	17.0		17.0		24.6
Unrealized loss, less than 12 months	0.0		0.0		0.0
Fair value, 12 months or greater	0.0		0.0		0.0
Unrealized loss, 12 months or greater	0.0		0.0		0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	17.0		17.0		24.6
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	0.0		0.0		0.0
Government-sponsored enterprise obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	51.1		51.1		113.8
Unrealized loss, less than 12 months	0.0		0.0		(0.1)
Fair value, 12 months or greater	0.0		0.0		0.0
Unrealized loss, 12 months or greater	0.0		0.0		0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	51.1		51.1		113.8
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	0.0		0.0		(0.1)
US government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	76.6		76.6		189.0
Unrealized loss, less than 12 months	0.0		0.0		(0.1)
Fair value, 12 months or greater	0.0		0.0		0.0
Unrealized loss, 12 months or greater	0.0		0.0		0.0
Total fair value, available-for-sale investments in continuous unrealized loss position	76.6		76.6		189.0
Total unrealized loss, available-for-sale investments in continuous unrealized loss position	$ 0.0		$ 0.0		(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					0.0
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)