Equity (Details Textual) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2011
Class of Stock [Line Items]
Payments for repurchase of common stock	$ 250.0	$ 583.6
Equity increase during the period	1,400.0
Net income attributable to Biogen Inc.	1,172.9	$ 747.6
2016 Share Repurchase Program
Class of Stock [Line Items]
Authorized amount of share repurchases under the 2016 Share Repurchase Program	$ 2,800.0		$ 5,000.0
Repurchase and retire of common stock, shares	0.9	0.8
Payments for repurchase of common stock	$ 250.0	$ 218.2
2011 Share Repurchase Program
Class of Stock [Line Items]
Common stock shares authorized for repurchased				20.0
Repurchase of common stock, shares		1.2
Payments for repurchase of common stock		$ 365.4