Equity (Details Textual) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Feb. 28, 2011
Equity (Textual) [Abstract]
Equity increase (decrease) during the period	$ 375,800,000
Net income attributable to Biogen Idec Inc	426,749,000	302,661,000
Purchase of treasury stock, value	$ 41,023,000	$ 463,171,000
Common stock shares authorized for repurchase	5.9		20.0
Repurchase of common stock, shares	0.3	(4.0)