Acquisitions (Details) (USD $)	12 Months Ended					12 Months Ended				12 Months Ended			12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 TYSABRI product	Apr. 02, 2013 TYSABRI product	Dec. 31, 2013 Stromedix, Inc.	Dec. 31, 2012 Stromedix, Inc.	Mar. 08, 2012 Stromedix, Inc.	Mar. 07, 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	Sep. 06, 2011 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH	Dec. 31, 2010 Biogen Idec Hemophilia	Dec. 31, 2013 Biogen Idec Hemophilia	Jan. 31, 2007 Biogen Idec Hemophilia	Mar. 08, 2012 STX One Hundred Stromedix, Inc.
Business Acquisition
Acquisition of TYSABRI rights	$ 3,262,719,000	$ 0	$ 0	$ 3,250,000,000
Deferred revenue	172,700,000	148,000,000			84,400,000
Net intangible asset					3,178,300,000
Estimated life, (In Years)				17 years
Future contingent payment for the first 12 months					12.00%
Future contingent payment for annual worldwide net sales up to $2.0 billion					18.00%
Future contingent payment threshold					2,000,000,000
Future contingent payment for annual worldwide net sales that exceed $2.0 billion					25.00%
Cash portion of consideration							75,000,000				152,900,000
Maximum contingent consideration in the form of development and approval milestones								487,500,000		38,500,000		42,500,000			80,000,000	275,000,000
Contingent consideration obligation	0	122,200,000					122,200,000				38,800,000
Total purchase price allocated to IPR&D								219,200,000
Total purchase price allocated to goodwill	1,232,916,000	1,201,296,000	1,146,300,000					48,200,000
Discount rate used to calculate fair value of in process research and development																20.00%
Other goodwill adjustments		6,800,000				(4,100,000)
Percentage of equity interest to the portion of total capital stock									5.00%
Recognized gain							9,000,000
Milestone payments made during period										4,000,000			40,000,000
Additional contingent payment for Biologic license														20,000,000
Additional contingent payment for marketing authorization														$ 20,000,000