Investments in Variable Interest Entities (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Knopp [Member]
Summary of activity related to collaboration
Total upfront and reimbursement payment made to collaborative partner		$ 26,400,000
Milestone payment payable to Acorda Therapeutics, Inc. (Acorda)	10,000,000
Total development expense incurred by Biogen Idec, excluding upfront and milestone payments	44,800,000	5,000,000
Biogen Idec's share of expense reflected within our consolidated statements of income	54,800,000	31,400,000
Collaboration expense attributed to noncontrolling interests, net of tax	8,600,000
Total upfront and milestone payments made to Collaborative Partner	36,400,000
Total expense incurred by Biogen Idec, excluding upfront and milestone payments	49,800,000
Estimate of additional amounts to be incurred by us in development of the lead compound	260,000,000
Neurimmune [Member]
Summary of activity related to collaboration
Milestone payment payable to Acorda Therapeutics, Inc. (Acorda)	15,000,000		7,500,000
Total development expense incurred by Biogen Idec, excluding upfront and milestone payments	9,000,000	15,500,000	9,000,000
Biogen Idec's share of expense reflected within our consolidated statements of income	24,000,000	15,500,000	16,500,000
Collaboration expense attributed to noncontrolling interests, net of tax	14,700,000	1,000,000
Total upfront and milestone payments made to Collaborative Partner	35,000,000
Total expense incurred by Biogen Idec, excluding upfront and milestone payments	40,000,000
Estimate of additional amounts to be incurred by us in development of the lead compound	$ 800,000,000