Equity (Details Textual) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Jul. 18, 2016	May 31, 2015
Equity [Abstract]
Equity increase during the period			$ 2,739.9
Net income attributable to Biogen Inc.	$ 1,032.9	$ 965.6	3,053.6	$ 2,715.4
Purchase of treasury stock			$ 348.9	$ 2,998.2
Authorized amount of share repurchases under the 2015 Share Repurchase Program					$ 5,000.0	$ 5,000.0
Common stock shares authorized for repurchased	1.3		1.3
Repurchase of common stock, shares			1.1	9.7
Milestone payment made	$ 0.0	$ 60.0	$ 0.0	$ 60.0