Equity (Details Textual) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2011
Class of Stock [Line Items]
Payments for repurchase of common stock			$ 1,365.4	$ 348.9
Equity increase during the period			720.3
Net income attributable to Biogen Inc.	$ 1,226.1	$ 1,032.9	2,836.5	3,053.6
Adjustment to retained earnings reflecting spin-off			852.8
2016 Share Repurchase Program [Member]
Class of Stock [Line Items]
Authorized amount of share repurchases under the 2015 Share Repurchase Program	$ 3,000.0	$ 5,000.0	$ 3,000.0	$ 5,000.0
Repurchase of common stock, shares			3.7	1.1
Payments for repurchase of common stock			$ 1,000.0	$ 348.9
2011 Share Repurchase Program [Member]
Class of Stock [Line Items]
Common stock shares authorized for repurchased					20.0
Repurchase of common stock, shares			1.3
Payments for repurchase of common stock			$ 365.4