Consolidated Statements of Equity - USD ($) $ in Millions	Total	Preferred stock	Common stock	Additional paid-in capital	Accumulated other comprehensive income (loss)	Retained earnings	Treasury stock	Noncontrolling interests	Parent
Beginning Balance at Dec. 31, 2013	$ (8,620.8)	$ 0.0	$ (0.1)	$ (4,023.6)	$ 27.7	$ (6,349.1)	$ (1,724.9)
Beginning Balance at Dec. 31, 2013	8,620.2
Beginning Balance at Dec. 31, 2013								$ 0.6
Beginning Balance, shares at Dec. 31, 2013		0	(256,000,000)				(19,700,000)
Net income attributable to Biogen Inc.	2,934.8					2,934.8
Net (loss) income attributable to noncontrolling interests, net of tax	6.8							6.8
Net income	2,941.6
Other comprehensive income (loss), net of tax	(31.8)				(31.8)
Other comprehensive income (loss), net of tax	(31.8)							0.0
Distribution or Capital contribution to noncontrolling interests	(9.1)							(9.1)	$ 0.0
Noncontrolling interest, increase (decrease) other	6.7							6.7	0.0
Repurchase of common stock, at cost	$ (886.8)						$ (886.8)
Repurchase of common stock, at cost, shares	(2,900,000)						(2,900,000)
Issuance of common stock under stock option and stock purchase plans	$ 54.9		$ 0.0	54.9
Issuance of common stock under stock option and stock purchase plans, shares			300,000
Issuance of common stock under stock award plan	(140.3)		$ 0.0	(140.3)
Issuance of common stock under stock award plan			800,000
Compensation expense related to share-based payments	165.0			165.0
Tax benefit from share-based payments	(93.0)			(93.0)
Ending Balance at Dec. 31, 2014	(10,814.0)	$ 0.0	$ (0.1)	(4,196.2)	(59.5)	(9,283.9)	$ (2,611.7)
Ending Balance at Dec. 31, 2014	10,809.0
Ending Balance at Dec. 31, 2014								5.0
Ending Balance, shares at Dec. 31, 2014		0	(257,100,000)				(22,600,000)
Net income attributable to Biogen Inc.	3,547.0					3,547.0
Net (loss) income attributable to noncontrolling interests, net of tax	46.2							46.2
Net income	3,593.2
Other comprehensive income (loss), net of tax	(164.5)				(164.5)
Other comprehensive income (loss), net of tax	164.5							0.0
Acquisition of noncontrolling interests	(10.9)							(10.9)	0.0
Distribution or Capital contribution to noncontrolling interests	(60.0)							60.0	0.0
Repurchase of common stock, at cost	$ (5,000.0)						$ (5,000.0)
Repurchase of common stock, at cost, shares	(16,800,000)						(16,800,000)
Retirement of common stock pursuant to the 2016 Share Repurchase Program, at cost	$ 0.0		$ 0.0	(4,377.5)		(622.5)	$ (5,000.0)
Retirement of common stock pursuant to the 2015 Share Repurchase Program, at cost, shares			(16,800,000)				(16,800,000)
Issuance of common stock under stock option and stock purchase plans	54.2		$ 0.0	54.2
Issuance of common stock under stock option and stock purchase plans, shares			300,000
Issuance of common stock under stock award plan	(125.1)		$ 0.0	(125.1)
Issuance of common stock under stock award plan			600,000
Compensation expense related to share-based payments	183.2			183.2
Tax benefit from share-based payments	(69.0)			(69.0)
Ending Balance at Dec. 31, 2015	(9,374.9)	$ 0.0	$ (0.1)	0.0	(224.0)	(12,208.4)	$ (2,611.7)
Ending Balance at Dec. 31, 2015	9,372.8
Ending Balance at Dec. 31, 2015	2.1							2.1
Ending Balance, shares at Dec. 31, 2015		0	(241,200,000)				(22,600,000)
Net income attributable to Biogen Inc.	3,702.8					3,702.8
Net (loss) income attributable to noncontrolling interests, net of tax	(7.1)							(7.1)
Net income	3,695.7
Other comprehensive income (loss), net of tax	(95.9)				(95.9)
Other comprehensive income (loss), net of tax	(95.8)							(0.1)
Acquisition of noncontrolling interests	(0.6)							0.6	0.0
Distribution or Capital contribution to noncontrolling interests	(1.5)							1.5	0.0
Deconsolidation of noncontrolling interests	(7.5)							(7.5)	$ 0.0
Repurchase of common stock, at cost	$ (1,000.0)						$ (1,000.0)
Repurchase of common stock, at cost, shares	(3,300,000)						(3,300,000)
Retirement of common stock pursuant to the 2016 Share Repurchase Program, at cost	$ 0.0		$ 0.0	(164.9)		(835.1)	$ (1,000.0)
Retirement of common stock pursuant to the 2015 Share Repurchase Program, at cost, shares			(3,300,000)				(3,300,000)
Issuance of common stock under stock option and stock purchase plans	43.7		$ 0.0	43.7		0.0
Issuance of common stock under stock option and stock purchase plans, shares			200,000
Issuance of common stock under stock award plan	$ (52.1)		$ 0.0	(47.6)		(4.5)
Issuance of common stock under stock award plan	41,000		400,000
Compensation expense related to share-based payments	$ 169.4			169.4
Tax benefit from share-based payments	(0.6)			0.6
Ending Balance at Dec. 31, 2016	(12,128.6)	$ 0.0	$ (0.1)	$ 0.0	$ (319.9)	$ (15,071.6)	$ (2,611.7)
Ending Balance at Dec. 31, 2016	12,140.1
Ending Balance at Dec. 31, 2016	$ (11.5)							$ (11.5)
Ending Balance, shares at Dec. 31, 2016		0	(238,500,000)				(22,600,000)