Fair Value Measurements (Details Textual) (USD $)	12 Months Ended					12 Months Ended				12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 6.0% Senior Notes due 2013 [Member]	Dec. 31, 2011 6.875% Senior Notes due 2018 [Member]	Dec. 31, 2011 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH [Member]	Sep. 06, 2011 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH [Member]	Dec. 31, 2011 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH [Member] Other Liabilities Current [Member]	Dec. 31, 2011 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH [Member] Other Liabilities Non Current [Member]	Dec. 31, 2011 Biogen Idec International Neuroscience GmbH [Member]	Dec. 31, 2010 Biogen Idec International Neuroscience GmbH [Member]	Dec. 31, 2011 Biogen Idec International Neuroscience GmbH [Member] Other Liabilities Current [Member]	Dec. 31, 2011 Biogen Idec International Neuroscience GmbH [Member] Other Liabilities Non Current [Member]
Investments and Acquisitions at Fair Value
Net cash outflow to determine valuations						$ 42,500,000				$ 390,000,000
Discount rate used for net cash outflow projections for fair value measurement						3.80%				5.20%
Fair value adjustment of contingent consideration	36,000,000					(6,900,000)				37,900,000
Payment to former shareholders of Syntonix Pharmaceuticals										5,000,000
Fair Value Measurements (Additional Textual) [Abstract]
Contingent consideration	$ 151,000,000	$ 81,200,000	$ 0			$ 31,900,000	$ 38,800,000	$ 3,900,000	$ 28,000,000	$ 119,100,000	$ 81,200,000	$ 6,900,000	$ 112,200,000
Debt Instrument [Line Items]
Interest rate on Senior Notes				6.00%	6.875%
Fair Value Measurements (Textual) [Abstract]
Venture capital investments as a percentage of our total assets	approximately 0.3% of total assets	approximately 0.3% of total assets
Percentage of venture capital investments to assets	0.30%