Equity (Details Textual) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Sep. 30, 2016	Mar. 31, 2011
Class of Stock [Line Items]
Payments for repurchase of common stock	$ 583.6	$ 0.0
Equity increase during the period	661.2
Adjustment to retained earnings reflecting spin-off	852.8
Net income attributable to Biogen Inc.	$ 747.6	$ 970.9
2016 Share Repurchase Program [Member]
Class of Stock [Line Items]
Authorized amount of share repurchases under the 2015 Share Repurchase Program			$ 5,000.0
Repurchase of common stock, shares	0.8
Payments for repurchase of common stock	$ 218.2
2016 Share Repurchase Program Plan To Date [Member]
Class of Stock [Line Items]
Repurchase of common stock, shares	4.1
Payments for repurchase of common stock	$ 1,200.0
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