Fair Value Measurements (Details Textual) (USD $) In Millions, unless otherwise specified	12 Months Ended				12 Months Ended			12 Months Ended				12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Other Liabilities Non Current	Dec. 31, 2012 Other Liabilities Non Current	Dec. 31, 2013 Stromedix, Inc.	Dec. 31, 2012 Stromedix, Inc.	Mar. 08, 2012 Stromedix, Inc.	Dec. 31, 2013 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH	Dec. 31, 2011 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH	Dec. 31, 2012 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH	Sep. 06, 2011 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH	Dec. 31, 2013 Biogen Idec International Neuroscience GmbH	Dec. 31, 2012 Biogen Idec International Neuroscience GmbH	Dec. 31, 2010 Biogen Idec International Neuroscience GmbH	Dec. 31, 2013 6.0% Senior Notes due 2013	Dec. 31, 2013 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%
Interest rate on senior notes															6.00%	6.875%
Contingent consideration obligations			$ 251.9	$ 271.5	$ 140.7	$ 135.3		$ 31.6		$ 29.8		$ 108.6	$ 128.8
Additions	0	122.2				122.2			38.8					81.2
Maximum contingent consideration in the form of development and approval milestones							487.5	38.5			42.5	375.0
Discount rate used for net cash outflow projections for fair value measurement					3.76%	4.40%		1.50%				3.80%
Milestone payments made during period								4.0				12.5	2.5
Total purchase price allocated to IPR&D							$ 219.2
Fair value measurements, changes in valuation techniques	0	0