Equity (Details Textual) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Feb. 28, 2011
Equity (Textual) [Abstract]
Equity increase during the period	$ 520,200,000
Net income attributable to Biogen Idec Inc	479,956,000	426,749,000
Adjustments to Additional Paid in Capital, Share-based Compensation and Exercise of Stock Options	31,100,000
Purchase of treasury stock, value	$ 0	$ 41,023,000
Common stock shares authorized for repurchase	4,200,000		20,000,000.0
Repurchase of common stock, shares		300,000