Equity (Details Textual) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Sep. 30, 2016	Mar. 31, 2011
Class of Stock [Line Items]
Payments for repurchase of common stock			$ 3,000.0	$ 1,365.4
Equity increase during the period			344.4
Net income attributable to Biogen Inc.	$ 866.6	$ 862.8	$ 2,039.5	$ 1,610.4
2016 Share Repurchase Program
Class of Stock [Line Items]
Authorized amount of share repurchases under the 2016 Share Repurchase Program						$ 5,000.0
Repurchase and retire of common stock, shares	9.6	2.9	10.5	3.7
Payments for repurchase of common stock	$ 2,750.0	$ 781.8	$ 3,000.0	$ 1,000.0
2011 Share Repurchase Program
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
Accounting Standards Update 2016-16 | Retained earnings
Class of Stock [Line Items]
Cumulative effect of new accounting principle					$ 500.0