Equity (Details) (USD $)	1 Months Ended		3 Months Ended		9 Months Ended
	Mar. 31, 2011	Feb. 28, 2011	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Equity (Textuals) [Abstract]
Equity increase (decrease) during the period					$ 784,200,000
Net income attributable to Biogen Idec Inc			351,826,000	254,115,000	934,190,000	764,995,000
Increase in additional capital from issuance of stock under our share based compensation					299,500,000
Reduction in additional paid in capital and noncontrolling interests					187,300,000
Total number of common stock shares issued on on conversion of Series A Preferred Stock	493,260
Preferred Stock, shares issued
Preferred Stock, shares outstanding
Common stock shares authorized for repurchase		20,000,000
Common Stock [Member]
Class of Stock [Line Items]
Repurchase of common stock, shares					5,000,000	40,300,000
Repurchase of common stock, value					$ 386,600,000	$ 2,100,000,000
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Series A Preferred Stock shares Redeemed	8,221