Cash Equivalents, and Investments - Unrealized Loss for Trading and Available for Sale Investments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 406.3	$ 282.5
Unrealized loss, less than 12 months	(0.7)	(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	406.3	282.6
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.7)	(0.3)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	403.7	279.6
Unrealized loss, less than 12 months	(0.4)	(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	403.7	279.7
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.4)	(0.2)
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	112.5	55.1	[1]
Unrealized loss, less than 12 months	(0.1)	(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	112.5	55.2	[1]
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.1)	(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	180.8	116.0
Unrealized loss, less than 12 months	(0.3)	(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	180.8	116.0
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.3)	(0.1)
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	15.1
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	15.1
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	85.3	30.0
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	85.3	30.0
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	0	0
US Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	10.0	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	10.0	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	2.6	2.9
Unrealized loss, less than 12 months	(0.3)	(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	2.6	2.9
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	$ (0.3)	$ (0.1)

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