Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Marketable Debt and Equity Securities
Fair Value	$ 3,700.9	$ 2,111.1
Gross Unrealized Gains	2.3	0.7
Gross Unrealized Losses	(2.2)	(2.4)
Amortized Cost	3,700.8	2,112.8
Corporate debt securities Current
Marketable Debt and Equity Securities
Fair Value	272.1	370.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.1)	(0.2)
Amortized Cost	272.2	370.6
Corporate debt securities Non-current
Marketable Debt and Equity Securities
Fair Value	767.8	692.6
Gross Unrealized Gains	0.7	0.2
Gross Unrealized Losses	(1.3)	(1.5)
Amortized Cost	768.4	693.9
Government securities Current
Marketable Debt and Equity Securities
Fair Value	1,481.4	269.9
Gross Unrealized Gains	0.6	0.0
Gross Unrealized Losses	0.0	(0.1)
Amortized Cost	1,480.8	270.0
Government securities Non-current
Marketable Debt and Equity Securities
Fair Value	929.7	579.9
Gross Unrealized Gains	0.8	0.3
Gross Unrealized Losses	(0.3)	(0.4)
Amortized Cost	929.2	580.0
Mortgage and other asset backed securities Current
Marketable Debt and Equity Securities
Fair Value	0.0	0.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Amortized Cost	0.0	0.2
Mortgage and other asset backed securities Non-current
Marketable Debt and Equity Securities
Fair Value	249.9	198.1
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	(0.5)	(0.2)
Amortized Cost	250.2	198.1
Marketable equity securities
Marketable Debt and Equity Securities
Fair Value	25.7	6.9
Gross Unrealized Gains	0.9	1.2
Gross Unrealized Losses	(3.4)	(0.2)
Amortized Cost	$ 28.2	$ 5.9