Equity (Details Textual) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jul. 18, 2016	May 31, 2015
Subsequent Event [Line Items]
Authorized amount of share repurchases under the 2015 Share Repurchase Program						$ 5,000.0
Equity increase during the period			$ 2,028.8
Net income attributable to Biogen Inc.	$ 1,049.8	$ 927.3	$ 2,020.7	$ 1,749.8
Common stock shares authorized for repurchased	1.3		1.3
Repurchase of common stock, shares				0.1
Purchase of treasury stock			$ 0.0	$ 42.2
Subsequent event
Subsequent Event [Line Items]
Authorized amount of share repurchases under the 2015 Share Repurchase Program					$ 5,000.0