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	March 2019 Share Repurchase Program	March 2019 Share Repurchase Program Common stock	March 2019 Share Repurchase Program Additional paid-in capital	March 2019 Share Repurchase Program Retained earnings	March 2019 Share Repurchase Program Treasury stock	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 Common stock	2016 Share Repurchase Program Additional paid-in capital	2016 Share Repurchase Program Retained earnings	2016 Share Repurchase Program Treasury stock	2011 Share Repurchase Program	2011 Share Repurchase Program Treasury stock
Beginning Balance at Dec. 31, 2016	$ 12,128.6	$ 0.0	$ 0.1	$ 0.0	$ (319.9)	$ 15,071.6	$ 2,611.7
Beginning Balance at Dec. 31, 2016	12,140.1
Beginning Balance at Dec. 31, 2016								$ (11.5)
Beginning 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
Capital contribution by noncontrolling interest	15.8							15.8	$ 0.0
Distribution to noncontrolling interests	(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)
Retirement of common stock pursuant to the 2018 Share Repurchase Program, at cost																				$ 0.0	$ 0.0	$ 36.0	$ 964.0	$ 1,000.0
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, shares			200,000
Issuance of common stock under stock options and stock purchase plans, value	40.5		$ 0.0	40.5
Issuance of common stock under stock award plan, shares			300,000
Issuance of common stock under stock award plan	(45.8)		$ 0.0	(44.8)		(1.0)
Hemophilia spin-off adjustment	(852.8)
Tax benefit	(17.5)					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)
Ending Balance, shares at Dec. 31, 2017		0	(235,300,000)				(23,800,000)
Net income attributable to Biogen Inc.	4,430.7					4,430.7
Net income (loss) attributable to noncontrolling interests, net of tax	43.3							43.3
Net income	4,474.0
Other comprehensive income (loss), net of tax	76.5				76.5
Other comprehensive income (loss), net of tax	(76.1)							(0.4)
Capital contribution by noncontrolling interest	13.8							13.8	0.0
Distribution to noncontrolling interests	(50.0)							(50.0)	0.0
Repurchase of common stock, at cost	(3,000.0)														$ (1,352.6)				$ (1,352.6)	$ (3,000.0)				$ (3,000.0)
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 2018 Share Repurchase Program, at cost															$ 0.0	$ 0.0	$ 92.8	$ 1,259.8	$ 1,352.6	$ 0.0	$ 0.0	$ 171.1	$ 2,828.9	$ 3,000.0
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, shares			200,000
Issuance of common stock under stock options and stock purchase plans, value	41.2		$ 0.0	41.2
Issuance of common stock under stock award plan, shares			300,000
Issuance of common stock under stock award plan	(43.8)		$ 0.0	(43.8)
Compensation related to share-based payments	(168.7)			(168.7)
Adoption of new accounting guidance	106.1				1.5	104.6
Ending Balance at Dec. 31, 2018	13,031.6	$ 0.0	$ 0.1	0.0	(240.4)	16,257.0	$ (2,977.1)
Ending Balance at Dec. 31, 2018	13,039.6
Ending Balance at Dec. 31, 2018	(8.0)							(8.0)
Ending Balance, shares at Dec. 31, 2018		0	(221,000,000.0)				(23,800,000)
Net income attributable to Biogen Inc.	5,888.5					5,888.5
Net income (loss) attributable to noncontrolling interests, net of tax	0.0							0.0
Net income	5,888.5
Other comprehensive income (loss), net of tax	105.2				105.2
Other comprehensive income (loss), net of tax	104.8							(0.4)
Capital contribution by noncontrolling interest	4.3							4.3	$ 0.0
Repurchase of common stock, at cost										$ (3,720.9)				$ (3,720.9)	$ (2,147.4)				$ (2,147.4)
Repurchase of common stock, at cost, shares										(14,700,000)				(14,700,000)	(8,900,000)				(8,900,000)
Retirement of common stock pursuant to the 2018 Share Repurchase Program, at cost										$ 0.0	$ 0.0	$ 121.5	$ 3,599.4	$ 3,720.9	$ 0.0	$ 0.0	$ 110.5	$ 2,036.9	$ 2,147.4
Retirement of common stock pursuant to the 2016 Share Repurchase Program, at cost, shares											14,700,000			14,700,000		8,900,000			8,900,000
Issuance of common stock under stock option and stock purchase plans, shares			200,000
Issuance of common stock under stock options and stock purchase plans, value	$ 40.8		$ 0.0	40.8		0.0
Issuance of common stock under stock award plan, shares	15,000		400,000
Issuance of common stock under stock award plan	$ (53.8)		$ 0.0	0.0		53.8
Compensation related to share-based payments	(191.2)			(191.2)
Ending Balance at Dec. 31, 2019	13,339.1	$ 0.0	$ 0.1	$ 0.0	$ (135.2)	$ 16,455.4	$ (2,977.1)
Ending Balance at Dec. 31, 2019	13,343.2
Ending Balance at Dec. 31, 2019	$ (4.1)							$ (4.1)
Ending Balance, shares at Dec. 31, 2019		0	(198,000,000.0)				(23,800,000)