Cash, Cash Equivalents and Marketable Securities Available-for-Sale (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Cash and cash equivalents
Money market funds	$ 860.4	$ 1,160.0
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,200.7	1,808.7
Gross Unrealized Gain	234.0	3.3
Gross Unrealized Loss	(10.4)	(2.1)
Estimated Fair Value	3,424.3	1,809.9
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,017.2	902.0
Gross Unrealized Gain	0.7	0.5
Gross Unrealized Loss	0
Estimated Fair Value	1,017.9	902.5
U.S. government-sponsored agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	400.1	303.5
Gross Unrealized Gain	0.2	0.3
Gross Unrealized Loss	(0.9)
Estimated Fair Value	399.4	303.8
U.S. government-sponsored agency MBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	587.1	387.2
Gross Unrealized Gain	0.5	1.6
Gross Unrealized Loss	(7.9)	(1.8)
Estimated Fair Value	579.7	387.0
Non-U.S. government, agency and Supranational securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	74.2	7.1
Gross Unrealized Loss	(0.1)
Estimated Fair Value	74.1	7.1
Corporate debt - global
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	701.8	208.5
Gross Unrealized Gain	0.9	0.9
Gross Unrealized Loss	(1.2)	(0.2)
Estimated Fair Value	701.5	209.2
Asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	229.6
Gross Unrealized Gain	0.1
Gross Unrealized Loss	(0.3)
Estimated Fair Value	229.4
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	190.7	0.4
Gross Unrealized Gain	231.6
Gross Unrealized Loss	0	(0.1)
Estimated Fair Value	$ 422.3	$ 0.3