Equity (Details Textual) - USD ($) $ in Thousands, shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Noncontrolling Interest [Line Items]
Net income (loss) attributable to NCI, net of tax	$ 53,871	$ (738)	$ 49,053	$ 7,660
Common stock shares repurchased			9.7	1.2
Purchase of treasury stock			$ 2,998,190	$ 359,981
Equity (Textual) [Abstract]
Equity increase during the period			247,100
Adjustments to additional paid in capital, share-based compensation and exercise of stock options			144,400
Net current period other comprehensive income loss	56,391	11,309	97,928	10,242
Net income attributable to Biogen Inc.	965,622	$ 856,858	2,715,439	$ 2,051,320
Common stock value authorized for repurchase	$ 5,000,000		$ 5,000,000
Common stock shares authorized remaining or repurchase	1.3		1.3
Additional paid-in capital	$ 1,342,373		$ 1,342,373		$ 4,196,156
Neurimmune
Noncontrolling Interest [Line Items]
Net income (loss) attributable to NCI, net of tax			$ 60,000