Equity (Details Textual) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Feb. 28, 2011
Equity (Textual) [Abstract]
Equity increase (decrease) during the period			$ 235,000,000
Net income attributable to Biogen Idec Inc	398,400,000	351,826,000	1,087,906,000	934,190,000
Increase in additional paid in capital, share based compensation			134,000,000
Repurchases of common stock			963,171,000	386,575,000
Common stock shares authorized for repurchase	6.3		6.3		20.0
Repurchase of common stock, shares	0.4		7.7	5.0
Repurchase of common stock, value	$ 53,200,000		$ 963,200,000	$ 386,600,000