Cash, Cash Equivalents, and Investments - Unrealized Loss for Trading and Available for Sale Investments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 599.4		$ 221.8
Unrealized loss, less than 12 months	(0.8)		(0.5)
Fair value, 12 months or greater	0.3		35.0
Unrealized loss, 12 months or greater	0		0
Total fair value, Available-for-sale investments in continuous unrealized loss position	599.7		256.8
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.8)		(0.5)
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	189.9		104.3	[1]
Unrealized loss, less than 12 months	(0.5)		(0.2)	[1]
Fair value, 12 months or greater	0		28.8	[1]
Unrealized loss, 12 months or greater	0		0	[1]
Total fair value, Available-for-sale investments in continuous unrealized loss position	189.9		133.1	[1]
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.5)		(0.2)	[1]
US Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	186.7
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	186.7
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.1)
US Government-sponsored Enterprises Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	146.0		57.8
Unrealized loss, less than 12 months	(0.1)		(0.2)
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	146.0		57.8
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.1)		(0.2)
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months			0	[1]
Unrealized loss, less than 12 months			0	[1]
Fair value, 12 months or greater			6.2	[1]
Unrealized loss, 12 months or greater			0	[1]
Total fair value, Available-for-sale investments in continuous unrealized loss position			6.2	[1]
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position			0	[1]
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months			5.0	[1]
Unrealized loss, less than 12 months			0	[1]
Fair value, 12 months or greater			0	[1]
Unrealized loss, 12 months or greater			0	[1]
Total fair value, Available-for-sale investments in continuous unrealized loss position			5.0	[1]
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position			0	[1]
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	76.8	[2]	54.7
Unrealized loss, less than 12 months	(0.1)	[2]	(0.1)
Fair value, 12 months or greater	0.3	[2]	0
Unrealized loss, 12 months or greater	0	[2]	0
Total fair value, Available-for-sale investments in continuous unrealized loss position	77.1	[2]	54.7
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	$ (0.1)	[2]	$ (0.1)

[1] Balance includes investments that were in an immaterial unrealized loss position as of December 31, 2010.

[2] Balance includes investments that were in an immaterial unrealized loss position as of December 31, 2011.