Investments in Variable Interest Entities (Details 1) (USD $)	6 Months Ended						3 Months Ended		6 Months Ended		1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 Corporate Joint Venture [Member]	Dec. 31, 2010 Corporate Joint Venture [Member]	Jun. 30, 2011 Corporate Joint Venture [Member] Biogen Dompe SRL [Member]	Dec. 31, 2010 Corporate Joint Venture [Member] Biogen Dompe SRL [Member]	Jun. 30, 2011 Neurimmune [Member]	Jun. 30, 2010 Neurimmune [Member]	Jun. 30, 2011 Neurimmune [Member]	Jun. 30, 2010 Neurimmune [Member]	Aug. 31, 2010 Knopp [Member]	Jun. 30, 2011 Knopp [Member]	Mar. 31, 2011 Knopp [Member]	Jun. 30, 2011 Knopp [Member]
Investment in Variable Interest Entities (Textuals) [Abstract]
Account receivable from ordinary course of business related to Joint Ventures	$ 666,960,000	$ 605,329,000	$ 134,200,000	$ 124,200,000	$ 127,800,000	$ 118,000,000
Total upfront and reimbursement payment made to collaborative partner											26,400,000
Remaining potential development milestone payments and royalties on commercial sales under the terms of collaboration agreement									345,000,000		265,000,000
Purchase of Class B Common Shares in Variable Interest Entities											30.00%
Purchase Consideration for Variable Interest Entities											60,000,000
Payment recognized as a charge to noncontrolling interests, net of tax									15,000,000				10,000,000
Biogen Idec's share of research and development expense reflected within our research and development expense							3,000,000	5,300,000	4,800,000	10,400,000		9,000,000		14,700,000
Percentage of funding for R&D cost required in support of the collaboration agreement									100.00%
Percentage of consolidated operations of sales affiliates	100.00%
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities	$ 17,700,000	$ 22,900,000