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	2016 Share Repurchase Program	2016 Share Repurchase Program Treasury stock	2011 Share Repurchase Program	2011 Share Repurchase Program Treasury stock
Beginning Balance at Dec. 31, 2014	$ (10,814.0)	$ 0.0	$ (0.1)	$ (4,196.2)	$ 59.5	$ (9,283.9)	$ (2,611.7)
Beginning Balance at Dec. 31, 2014	10,809.0
Beginning Balance at Dec. 31, 2014								$ 5.0
Beginning 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 income (loss) 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 interest	(10.9)							(10.9)	$ 0.0
Distribution to noncontrolling interest	(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			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 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
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 income (loss) 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)
Noncontrolling interest, increase (decrease) other	1.5							1.5	0.0
Acquisition of noncontrolling interest	(0.6)							(0.6)	0.0
Deconsolidation of noncontrolling interest	(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
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			400,000
Compensation 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)
Net income attributable to Biogen Inc.	2,539.1					2,539.1
Net income (loss) attributable to noncontrolling interests, net of tax	131.0							131.0
Net income	2,670.1
Other comprehensive income (loss), net of tax	1.5				1.5
Other comprehensive income (loss), net of tax	1.5							0.0
Noncontrolling interest, increase (decrease) other	15.8							15.8	0.0
Distribution to noncontrolling interest	(150.0)							(150.0)	$ 0.0
Repurchase of common stock, at cost										$ (1,000.0)	$ (1,000.0)	$ (365.4)	$ (365.4)
Repurchase of common stock, at cost, shares										(3,700,000)	(3,700,000)	(1,200,000)	(1,200,000)
Retirement of common stock pursuant to the 2016 Share Repurchase Program, at cost	0.0		$ 0.0	36.0		964.0	$ 1,000.0
Retirement of common stock pursuant to the 2015 Share Repurchase Program, at cost, shares			3,700,000				3,700,000
Issuance of common stock under stock option and stock purchase plans	40.5		$ 0.0	40.5		0.0
Issuance of common stock under stock option and stock purchase plans, shares			200,000
Issuance of common stock under stock award plan	$ (45.8)		$ 0.0	(44.8)		(1.0)
Issuance of common stock under stock award plan	14,000		300,000
Compensation related to share-based payments	$ 138.1			138.1
Hemophilia spin-off adjustment	(852.8)					(852.8)
Tax benefit	17.5					17.5
Ending Balance at Dec. 31, 2017	(12,598.1)	$ 0.0	$ (0.1)	$ (97.8)	$ (318.4)	$ (15,810.4)	$ (2,977.1)
Ending Balance at Dec. 31, 2017	12,612.8
Ending Balance at Dec. 31, 2017	$ (14.7)							$ (14.7)
Ending Balance, shares at Dec. 31, 2017		0	(235,300,000)				(23,800,000)