Fair Value Measurements (Details Textual) (USD $) In Millions, unless otherwise specified	12 Months Ended					12 Months Ended			3 Months Ended	12 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Other Liabilities Non Current	Dec. 31, 2011 Other Liabilities Non Current	Dec. 31, 2012 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH	Dec. 31, 2011 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH	Sep. 06, 2011 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH	Mar. 31, 2012 Stromedix, Inc.	Dec. 31, 2012 Stromedix, Inc.	Mar. 08, 2012 Stromedix, Inc.	Dec. 31, 2012 Biogen Idec International Neuroscience GmbH	Dec. 31, 2011 Biogen Idec International Neuroscience GmbH	Dec. 31, 2010 Biogen Idec International Neuroscience GmbH	Dec. 31, 2012 6.0% Senior Notes due 2013	Dec. 31, 2012 6.875% Senior Notes due 2018
Business Acquisition, Contingent Consideration [Line Items]
Venture capital investments as percentage of our assets	approximately 0.2% of total assets	approximately 0.2% of total assets
Percentage of venture capital investments to assets	0.20%	0.20%
Contingent consideration obligations	$ 293.9	$ 151.0	$ 81.2	$ 271.5	$ 140.3	$ 29.8	$ 31.9	$ 38.8		$ 135.3	$ 122.2	$ 128.8	$ 119.1	$ 81.2
Value of contingent consideration						38.5		42.5			487.5	387.5		395.0
Discount rate used for net cash outflow projections for fair value measurement						2.40%			4.40%	3.60%		3.60%
Business combination, contingent consideration arrangements, change in amount of contingent consideration, liability	(27.2)	(36.0)				9.0				13.1		(47.6)
Milestone payments made during period						4.0						7.5
In-process research and development											219.2			110.9
Interest rate on senior notes															6.00%	6.875%
Impairment of long-lived assets held-for-use	$ 0	$ 0
Fair value measurements, changes in valuation techniques	0	0