Acquisitions (Details) (USD $)						12 Months Ended		12 Months Ended				12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	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 [Member]	Sep. 06, 2011 Biogen Dompe SRL [Member]	Dec. 31, 2010 Biogen Idec Hemophilia [Member]	Dec. 31, 2011 Biogen Idec Hemophilia [Member]	Dec. 31, 2007 Biogen Idec Hemophilia [Member]	Jan. 31, 2007 Biogen Idec Hemophilia [Member]	Dec. 31, 2011 Biogen Idec International Neuroscience GmbH [Member]	Dec. 31, 2010 Biogen Idec International Neuroscience GmbH [Member]
Acquisitions (Textual) [Abstract]
Cash portion of consideration					$ 152,900,000								$ 32,500,000
Value of contingent consideration					42,500,000								395,000,000
Stock acquired in acquisition											100.00%		100.00%
Receivables purchased as part of acquisition						104,600,000	104,600,000
Loss recognized upon disposition						1,800,000
Fair value of contingent consideration	151,000,000	81,200,000	0	31,900,000	38,800,000					80,000,000		119,100,000	81,200,000
Payment to former shareholders of Syntonix Pharmaceuticals								40,000,000				5,000,000
Additional contingent payment for Biologic License									20,000,000
Additional contingent payment for marketing authorization									20,000,000
In process research and development													110,900,000
Goodwill													$ 25,600,000