Consolidated Statements of Equity - USD ($)	Total	Preferred stock	Common stock	Additional paid-in capital	Accumulated other comprehensive income (loss)	Retained earnings	Treasury stock	Noncontrolling interests	Parent	2018 Share Repurchase Program	2018 Share Repurchase Program Common stock	2018 Share Repurchase Program Additional paid-in capital	2018 Share Repurchase Program Retained earnings	2018 Share Repurchase Program Treasury stock	2016 Share Repurchase Program	2016 Share Repurchase Program Treasury stock	2011 Share Repurchase Program	2011 Share Repurchase Program Treasury stock	2016 Share Repurchase Program	2016 Share Repurchase Program Common stock	2016 Share Repurchase Program Additional paid-in capital	2016 Share Repurchase Program Retained earnings	2016 Share Repurchase Program Treasury stock	Accounting Standards Update 2016-01 Retained earnings	Accounting Standards Update 2016-16 Retained earnings
Beginning Balance at Dec. 31, 2015	$ 9,374,900,000	$ 0	$ 100,000	$ 0	$ (224,000,000)	$ 12,208,400,000	$ 2,611,700,000
Beginning Balance at Dec. 31, 2015	9,372,800,000
Beginning Balance at Dec. 31, 2015								$ 2,100,000
Beginning Balance, shares at Dec. 31, 2015		0	(241,200,000)				(22,600,000)
Net income attributable to Biogen Inc.	3,702,800,000					3,702,800,000
Net income (loss) attributable to noncontrolling interests, net of tax	(7,100,000)							(7,100,000)
Net income	3,695,700,000
Other comprehensive income (loss), net of tax	(95,900,000)				(95,900,000)
Other comprehensive income (loss), net of tax	(95,800,000)							100,000
Acquisition of noncontrolling interest	600,000							600,000	$ 0
Capital contribution by noncontrolling interest	1,500,000							1,500,000	0
Deconsolidation of noncontrolling interest	(7,500,000)								0
Repurchase of common stock, at cost	(1,000,000,000)						$ (1,000,000,000)
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	(164,900,000)		(835,100,000)	$ 1,000,000,000
Retirement of common stock pursuant to the 2016 Share Repurchase Program, at cost, shares			(3,300,000)				(3,300,000)
Issuance of common stock under stock option and stock purchase plans	43,700,000		$ 0	43,700,000
Issuance of common stock under stock option and stock purchase plans, shares			200,000
Issuance of common stock under stock award plan	(52,100,000)		$ 0	(47,600,000)		(4,500,000)
Issuance of common stock under stock award plan			400,000
Compensation related to share-based payments	169,400,000			169,400,000
Tax benefit from share-based payments	(600,000)			600,000					(600,000)
Ending Balance at Dec. 31, 2016	12,128,600,000	$ 0	$ 100,000	0	(319,900,000)	15,071,600,000	$ (2,611,700,000)
Ending Balance at Dec. 31, 2016	12,140,100,000
Ending Balance at Dec. 31, 2016								(11,500,000)
Ending Balance, shares at Dec. 31, 2016		0	(238,500,000)				(22,600,000)
Net income attributable to Biogen Inc.	2,539,100,000					2,539,100,000
Net income (loss) attributable to noncontrolling interests, net of tax	131,000,000							131,000,000
Net income	2,670,100,000
Other comprehensive income (loss), net of tax	1,500,000				1,500,000
Other comprehensive income (loss), net of tax	(1,500,000)							0
Capital contribution by noncontrolling interest	15,800,000							15,800,000	0
Distribution to noncontrolling interest	150,000,000							150,000,000	0
Repurchase of common stock, at cost															$ (1,000,000,000)	$ (1,000,000,000)	$ (365,400,000)	$ (365,400,000)
Repurchase of common stock, at cost, shares																(3,700,000)	(1,200,000)	(1,200,000)	(3,700,000)
Retirement of common stock pursuant to the 2016 Share Repurchase Program, at cost	0		$ 0	(36,000,000)		(964,000,000)	$ (1,000,000,000)
Retirement of common stock pursuant to the 2016 Share Repurchase Program, at cost, shares			(3,700,000)				(3,700,000)
Issuance of common stock under stock option and stock purchase plans	40,500,000		$ 0	40,500,000
Issuance of common stock under stock option and stock purchase plans, shares			200,000
Issuance of common stock under stock award plan	(45,800,000)		$ 0	(44,800,000)		(1,000,000)
Issuance of common stock under stock award plan			300,000
Compensation related to share-based payments	(138,100,000)			(138,100,000)
Hemophilia spin-off adjustment	(852,800,000)					(852,800,000)
Tax benefit	17,500,000					17,500,000
Ending Balance at Dec. 31, 2017	12,598,100,000	$ 0	$ 100,000	97,800,000	(318,400,000)	15,810,400,000	$ (2,977,100,000)
Ending Balance at Dec. 31, 2017	12,612,800,000
Ending Balance at Dec. 31, 2017	(14,700,000)							(14,700,000)
Ending Balance, shares at Dec. 31, 2017		0	(235,300,000)				(23,800,000)
Net income attributable to Biogen Inc.	4,430,700,000					4,430,700,000
Net income (loss) attributable to noncontrolling interests, net of tax	43,300,000							43,300,000
Net income	4,474,000,000
Other comprehensive income (loss), net of tax	76,500,000				76,500,000
Other comprehensive income (loss), net of tax	76,100,000							(400,000)
Capital contribution by noncontrolling interest	13,800,000							13,800,000	0
Distribution to noncontrolling interest	50,000,000							50,000,000	$ 0
Repurchase of common stock, at cost										$ (1,352,600,000)				$ (1,352,600,000)	$ (3,000,000,000)	$ (3,000,000,000)
Repurchase of common stock, at cost, shares										(4,300,000)				(4,300,000)		(10,500,000)			(10,500,000)
Retirement of common stock pursuant to the 2016 Share Repurchase Program, at cost										$ 0	$ 0	$ (92,800,000)	$ (1,259,800,000)	$ (1,352,600,000)					$ 0	$ 0	$ (171,100,000)	$ (2,828,900,000)	$ (3,000,000,000)
Retirement of common stock pursuant to the 2016 Share Repurchase Program, at cost, shares											(4,300,000)			(4,300,000)						(10,500,000)			(10,500,000)
Issuance of common stock under stock option and stock purchase plans	41,200,000		$ 0	41,200,000
Issuance of common stock under stock option and stock purchase plans, shares			200,000
Issuance of common stock under stock award plan	$ (43,800,000)		$ 0	(43,800,000)
Issuance of common stock under stock award plan	15,000		300,000
Compensation related to share-based payments	$ (168,700,000)			(168,700,000)
Tax benefit	106,100,000																							$ 1,500,000	$ 104,600,000
Ending Balance at Dec. 31, 2018	13,031,600,000	$ 0	$ 100,000	$ 0	$ (240,400,000)	$ 16,257,000,000	$ (2,977,100,000)
Ending Balance at Dec. 31, 2018	13,039,600,000
Ending Balance at Dec. 31, 2018	$ (8,000,000)							$ (8,000,000)
Ending Balance, shares at Dec. 31, 2018		0	(221,000,000)				(23,800,000)