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Fair Value Measurements (Details Textuals) (USD $) In Millions, unless otherwise specified						3 Months Ended	9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 06, 2011 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH [Member]	Sep. 06, 2011 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH [Member] Other Liabilities Current [Member]	Sep. 06, 2011 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH [Member] Other Liabilities Non Current [Member]	Sep. 30, 2011 Biogen Idec International Neuroscience GmbH [Member]	Sep. 30, 2011 Biogen Idec International Neuroscience GmbH [Member]	Dec. 17, 2011 Biogen Idec International Neuroscience GmbH [Member]	Sep. 30, 2011 Biogen Idec International Neuroscience GmbH [Member] Other Liabilities Current [Member]	Sep. 30, 2011 Biogen Idec International Neuroscience GmbH [Member] Other Liabilities Non Current [Member]	Sep. 30, 2011 6.0% Senior Notes due 2013 [Member]	Sep. 30, 2011 6.875% Senior Notes due 2018 [Member]
Investments and Acquisitions at Fair Value
Net cash outflow to determine valuations			$ 42.5			$ 395.0	$ 395.0
Discount rate used for net cash outflow projections for fair value measurement			3.20%			5.30%	5.30%
Fair value adjustment of contingent consideration						2.5	5.9
Fair Value Measurements (Additional Textuals) [Abstract]
Contingent consideration			$ 38.8	$ 4.0	$ 34.8	$ 87.1	$ 87.1	$ 81.2	$ 5.0	$ 82.1
Fair Value, Option, Quantitative Disclosures [Line Items]
Interest rate on Senior Notes											6.00%	6.875%
Fair Value Measurements (Textuals) [Abstract]
Venture capital investments as a percentage of our total assets	approximately 0.3% of total assets	approximately 0.3% of total assets