Cash, Cash Equivalents, and Investments - Unrealized Loss for Trading and Available for Sale Investments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 409.4		$ 599.4
Unrealized loss, less than 12 months	(0.5)		(0.8)
Fair value, 12 months or greater	0.9		0.3
Unrealized loss, 12 months or greater	0		0
Total fair value, Available-for-sale investments in continuous unrealized loss position	410.3		599.7
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.5)		(0.8)
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	145.6		189.9
Unrealized loss, less than 12 months	(0.2)		(0.5)
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	145.6		189.9
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.2)		(0.5)
US Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	194.1		186.7
Unrealized loss, less than 12 months	(0.1)		(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	194.1		186.7
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	(0.1)		(0.1)
Government-Sponsored Enterprise Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	31.0	[1]	146.0
Unrealized loss, less than 12 months	0	[1]	(0.1)
Fair value, 12 months or greater	0	[1]	0
Unrealized loss, 12 months or greater	0	[1]	0
Total fair value, Available-for-sale investments in continuous unrealized loss position	31.0	[1]	146.0
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	0	[1]	(0.1)
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	35.9	[1]	76.8	[2]
Unrealized loss, less than 12 months	0	[1]	(0.1)	[2]
Fair value, 12 months or greater	0.9	[1]	0.3	[2]
Unrealized loss, 12 months or greater	0	[1]	0	[2]
Total fair value, Available-for-sale investments in continuous unrealized loss position	36.8	[1]	77.1	[2]
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	0	[1]	(0.1)	[2]
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	2.8
Unrealized loss, less than 12 months	(0.2)
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	2.8
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	$ (0.2)

[1] Balance includes investments that were in an immaterial unrealized loss position as of June 30, 2012.

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