Investments in Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Neurimmune
Summary of activity related to collaboration
Milestone payments made during period	$ 0	$ 0	$ 15.0
Biogen Idec's share of expense reflected within our consolidation statements of income	27.2	13.3	24.2
Collaboration expense attributed to noncontrolling interests, net of tax	0	0	14.7
Total expense incurred by Biogen Idec	115.7
Estimate of additional amounts to be incurred by us in development of the lead compound	800.9
Knopp
Summary of activity related to collaboration
Milestone payments made during period	0	0	10.0
Biogen Idec's share of expense reflected within our consolidation statements of income	0	113.0	54.8
Collaboration expense attributed to noncontrolling interests, net of tax	$ 0	$ 0	$ 8.6