Cash, Cash Equivalents and Marketable Securities Available-for-Sale (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Cash and cash equivalents
Money market funds	$ 643.1	$ 1,160.0
Available-for-sale securities by major security type and class of security
Amortized Cost	2,024.9	1,808.7
Gross Unrealized Gain	3.6	3.3
Gross Unrealized Loss	(3.6)	(2.1)
Estimated Fair Value	2,024.9	1,809.9
U.S. Treasury securities
Available-for-sale securities by major security type and class of security
Amortized Cost	848.6	902.0
Gross Unrealized Gain	0.6	0.5
Estimated Fair Value	849.2	902.5
U.S. government-sponsored agency securities
Available-for-sale securities by major security type and class of security
Amortized Cost	232.9	303.5
Gross Unrealized Gain	0.3	0.3
Estimated Fair Value	233.2	303.8
U.S. government-sponsored agency MBS
Available-for-sale securities by major security type and class of security
Amortized Cost	558.2	387.2
Gross Unrealized Gain	1.7	1.6
Gross Unrealized Loss	(2.9)	(1.8)
Estimated Fair Value	557.0	387.0
Non-U.S. government, agency and Supranational securities
Available-for-sale securities by major security type and class of security
Amortized Cost	10.4	7.1
Estimated Fair Value	10.4	7.1
Corporate debt - global
Available-for-sale securities by major security type and class of security
Amortized Cost	336.3	208.5
Gross Unrealized Gain	1.0	0.9
Gross Unrealized Loss	(0.5)	(0.2)
Estimated Fair Value	336.8	209.2
Asset backed securities
Available-for-sale securities by major security type and class of security
Amortized Cost	38.1
Estimated Fair Value	38.1
Marketable equity securities
Available-for-sale securities by major security type and class of security
Amortized Cost	0.4	0.4
Gross Unrealized Loss	(0.2)	(0.1)
Estimated Fair Value	$ 0.2	$ 0.3