Cash Equivalents and Investments - Unrealized Loss on Available-for-Sale Securities (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 703.2	$ 282.5
Unrealized loss, less than 12 months	(2.2)	(0.3)
Fair value, 12 months or greater	0	0.1
Unrealized loss, 12 months or greater	0	0
Total fair value, Available-for-sale investments in continuous unrealized loss position	703.2	282.6
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(2.2)	(0.3)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	700.4	279.6
Unrealized loss, less than 12 months	(2.0)	(0.2)
Fair value, 12 months or greater	0	0.1
Unrealized loss, 12 months or greater	0	0
Total fair value, Available-for-sale investments in continuous unrealized loss position	700.4	279.7
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(2.0)	(0.2)
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	195.3	55.1	[1]
Unrealized loss, less than 12 months	(0.5)	(0.1)	[1]
Fair value, 12 months or greater	0	0.1	[1]
Unrealized loss, 12 months or greater	0	0	[1]
Total fair value, Available-for-sale investments in continuous unrealized loss position	195.3	55.2	[1]
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.5)	(0.1)	[1]
Certificate of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months		0.3
Unrealized loss, less than 12 months		0
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		0.3
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position		0
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months		10.0
Unrealized loss, less than 12 months		0
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		10.0
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position		0
Corporate debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	304.0	116.0
Unrealized loss, less than 12 months	(1.0)	(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	304.0	116.0
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(1.0)	(0.1)
Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	15.0
Unrealized loss, less than 12 months	0
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	15.0
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	0
Government-sponsored enterprise obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	129.6	30.0
Unrealized loss, less than 12 months	(0.5)	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	129.6	30.0
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.5)	0
US government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	56.5	68.2
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	56.5	68.2
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	0	0
Publicly-traded equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	2.8	2.9
Unrealized loss, less than 12 months	(0.2)	(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	2.8	2.9
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	$ (0.2)	$ (0.1)

[1] Balance greater than 12 months includes investments that were in an immaterial unrealized loss position as of December 31, 2012.