Equity (Details Textual) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Sep. 30, 2016	Mar. 31, 2011
Class of Stock [Line Items]
Payments for repurchase of common stock			$ 1,365.4	$ 0.0
Equity increase during the period			555.0
Adjustment to retained earnings reflecting spin-off			852.8
Net income attributable to Biogen Inc.	$ 862.8	$ 1,049.8	1,610.4	$ 2,020.7
2016 Share Repurchase Program [Member]
Class of Stock [Line Items]
Authorized amount of share repurchases under the 2015 Share Repurchase Program	$ 3,000.0		$ 3,000.0		$ 5,000.0
Repurchase of common stock, shares	2.9		3.7
Payments for repurchase of common stock	$ 781.8		$ 1,000.0
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