Equity (Details Textual) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Feb. 28, 2011
Equity (Textual) [Abstract]
Equity increase (decrease) during the period			$ 938,400,000
Net income attributable to Biogen Idec Inc	490,670,000	386,846,000	917,419,000	689,507,000
Adjustments to Additional Paid in Capital, Share-based Compensation and Exercise of Stock Options			63,500,000
Purchase of treasury stock, value			$ 41,023,000	$ 909,951,000
Common stock shares authorized for repurchase	5.9		5.9		20.0
Repurchase of common stock, shares			0.3	7.3