Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Marketable Debt and Equity Securities
Fair Value	$ 382.3	$ 3,171.7
Gross Unrealized Gains	0	5.6
Gross Unrealized Losses	0.2	1.9
Amortized Cost	382.5	3,168.0
Corporate debt securities Current [Member]
Marketable Debt and Equity Securities
Fair Value	26.4	346.9
Gross Unrealized Gains	0	0.3
Gross Unrealized Losses	0	0
Amortized Cost	26.4	346.6
Corporate debt securities Non-current [Member]
Marketable Debt and Equity Securities
Fair Value	73.2	654.1
Gross Unrealized Gains	0	2.8
Gross Unrealized Losses	0.2	0.6
Amortized Cost	73.4	651.9
Government securities Current [Member]
Marketable Debt and Equity Securities
Fair Value	248.8	783.4
Gross Unrealized Gains	0	0.3
Gross Unrealized Losses	0	0
Amortized Cost	248.8	783.1
Government securities Non-current [Member]
Marketable Debt and Equity Securities
Fair Value	24.4	874.4
Gross Unrealized Gains	0	0.8
Gross Unrealized Losses	0	0
Amortized Cost	24.4	873.6
Mortgage and other asset backed securities Current [Member]
Marketable Debt and Equity Securities
Fair Value	0	4.8
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Amortized Cost	0	4.8
Mortgage and other asset backed securities Non-current [Member]
Marketable Debt and Equity Securities
Fair Value	9.5	508.1
Gross Unrealized Gains	0	1.4
Gross Unrealized Losses	0	1.3
Amortized Cost	9.5	508.0
Marketable equity securities [Member]
Marketable Debt and Equity Securities
Gross Unrealized Gains	14.7	3.0
Gross Unrealized Losses	0	0
Amortized Cost	14.2	6.0
Fair Value	$ 28.9	$ 9.0