Equity (Details Textual) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Feb. 28, 2011
Equity (Textual) [Abstract]
Equity increase during the period	$ 855,400,000
Net income attributable to Biogen Inc	822,541,000	479,956,000
Adjustments to additional paid in capital, share-based compensation and exercise of stock options	$ 46,300,000
Common stock shares authorized for repurchase	1,300,000		20,000,000.0