Equity (Details Textual) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Feb. 28, 2011
Equity (Textual) [Abstract]
Equity increase during the period			$ 1,810,000,000
Net income attributable to Biogen Idec Inc	856,858,000	487,616,000	2,051,320,000	1,405,035,000
Adjustments to additional paid in capital, share-based compensation and exercise of stock options			126,400,000
Common stock shares authorized for repurchase	2,900,000		2,900,000		20,000,000.0
Repurchase of common stock, shares			1,200,000	2,000,000
Purchase of treasury stock, value			$ 359,981,000	$ 400,308,000