Other Consolidated Financial Statement Detail (Details Textual) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 New Cambridge Leases [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 Idec International Neuroscience GmbH [Member]	Dec. 31, 2010 Biogen Idec International Neuroscience GmbH [Member]
Other Consolidated Financial Statement Detail (Textual) [Abstract]
In process research and development								$ 110.9
Goodwill								25.6
Fair value of contingent consideration	151.0	81.2	0		31.9	38.8	119.1	81.2
Deferred tax liability assumed								23.7
Remaining obligation of financing arrangements				$ 2.2