Cash, Cash Equivalents, and Investments - Unrealized Loss for Trading and Available for Sale Investments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	$ 128.7	$ 599.4
Unrealized loss, less than 12 months	0	(0.8)
Fair value, 12 months or greater	1.4	0.3
Unrealized loss, 12 months or greater	0	0
Total fair value, Available-for-sale investments in continuous unrealized loss position	130.1	599.7
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	0	(0.8)
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	40.8	189.9
Unrealized loss, less than 12 months	0	(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	40.8	189.9
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	0	(0.5)
US Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	51.0	186.7
Unrealized loss, less than 12 months	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	51.0	186.7
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	0	(0.1)
Government-Sponsored Enterprise Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	20.0	146.0
Unrealized loss, less than 12 months	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	20.0	146.0
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	0	(0.1)
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than 12 months	16.9	76.8	[1]
Unrealized loss, less than 12 months	0	(0.1)	[1]
Fair value, 12 months or greater	1.4	0.3	[1]
Unrealized loss, 12 months or greater	0	0	[1]
Total fair value, Available-for-sale investments in continuous unrealized loss position	18.3	77.1	[1]
Total unrealized loss, Available-for-sale investments in continuous unrealized loss position	$ 0	$ (0.1)	[1]

[1] Balance greater than 12 months includes investments that were in an immaterial unrealized loss position as of December 31, 2011.