Equity (Details Textual) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Feb. 28, 2011
Equity (Textual) [Abstract]
Equity increase during the period			$ 1,161,900,000
Net income attributable to Biogen Idec Inc	487,616,000	398,400,000	1,405,035,000	1,087,906,000
Adjustments to Additional Paid in Capital, Share-based Compensation and Exercise of Stock Options			132,900,000
Purchase of treasury stock, value	$ (359,300,000)		$ 400,308,000	$ 963,171,000
Common stock shares authorized for repurchase	4.2		4.2		20.0
Repurchase of common stock, shares	1.7		2.0	7.7