Consolidated Statements of Equity - USD ($) shares in Millions, $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Preferred stock	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated other comprehensive loss Cumulative Effect, Period of Adoption, Adjustment	Retained earnings	Retained earnings Cumulative Effect, Period of Adoption, Adjustment	Treasury stock	Total Biogen Inc. shareholders’ equity	Total Biogen Inc. shareholders’ equity Cumulative Effect, Period of Adoption, Adjustment	Noncontrolling interests	2020 Share Repurchase Program	2020 Share Repurchase Program Common stock	2020 Share Repurchase Program Additional paid-in capital	2020 Share Repurchase Program Retained earnings	2020 Share Repurchase Program Treasury stock	2020 Share Repurchase Program Total Biogen Inc. shareholders’ equity	December 2019 Share Repurchase Program	December 2019 Share Repurchase Program Common stock	December 2019 Share Repurchase Program Additional paid-in capital	December 2019 Share Repurchase Program Retained earnings	December 2019 Share Repurchase Program Treasury stock	December 2019 Share Repurchase Program Total Biogen Inc. shareholders’ equity	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	March 2019 Share Repurchase Program Total Biogen Inc. shareholders’ equity	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	2018 Share Repurchase Program Total Biogen Inc. shareholders’ equity	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	2016 Share Repurchase Program Total Biogen Inc. shareholders’ equity
Beginning balance (in shares) at Dec. 31, 2017			0.0	235.3						23.8
Beginning balance at Dec. 31, 2017	$ 12,598.1	$ 106.1	$ 0.0	$ 0.1	$ 97.8	$ (318.4)	$ 1.5	$ 15,810.4	$ 104.6	$ (2,977.1)	$ 12,612.8	$ 106.1	$ (14.7)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	4,474.0							4,430.7			4,430.7		43.3
Other comprehensive income (loss), net of tax	76.1					76.5					76.5		(0.4)
Capital contribution from noncontrolling interest	13.8										0.0		13.8
Distribution to noncontrolling interests	(50.0)										0.0		(50.0)
Repurchase of common stock pursuant to the share repurchase program, at cost (in shares)																																(4.3)				(4.3)		(10.5)				(10.5)
Repurchase of common stock pursuant to the share repurchase program, at cost																																$ (1,352.6)				$ (1,352.6)	$ (1,352.6)	$ (3,000.0)				$ (3,000.0)	$ (3,000.0)
Retirement of common stock pursuant to the share repurchase program, at cost (in shares)																																	4.3			4.3			10.5			10.5
Retirement of common stock pursuant to the share repurchase program, at cost																																$ 0.0	$ 0.0	$ (92.8)	$ (1,259.8)	$ 1,352.6	0.0	$ 0.0	$ 0.0	$ (171.1)	$ (2,828.9)	$ 3,000.0	$ 0.0
Issuance of common stock under stock option and stock purchase plans (in shares)				0.2
Issuance of common stock under stock option and stock purchase plans	41.2			$ 0.0	41.2						41.2
Issuance of common stock under stock award plan (in shares)				0.3
Issuance of common stock under stock award plan	(43.8)			$ 0.0	(43.8)						(43.8)
Compensation expense related to share-based payments	168.7				168.7						168.7
Ending balance (in shares) at Dec. 31, 2018			0.0	221.0						23.8
Ending balance at Dec. 31, 2018	13,031.6		$ 0.0	$ 0.1	0.0	(240.4)		16,257.0		$ (2,977.1)	13,039.6		(8.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	5,888.5							5,888.5			5,888.5
Other comprehensive income (loss), net of tax	104.8					105.2					105.2		(0.4)
Capital contribution from noncontrolling interest	4.3										0.0		4.3
Repurchase of common stock pursuant to the share repurchase program, at cost (in shares)																										(14.7)				(14.7)		(8.9)				(8.9)
Repurchase of common stock pursuant to the share repurchase program, at cost																										$ (3,720.9)				$ (3,720.9)	$ (3,720.9)	$ (2,147.4)				$ (2,147.4)	(2,147.4)
Retirement of common stock pursuant to the share repurchase program, at cost (in shares)																											14.7			14.7			8.9			8.9
Retirement of common stock pursuant to the share repurchase program, at cost																										$ 0.0	$ 0.0	$ (121.5)	$ (3,599.4)	$ 3,720.9	0.0	$ 0.0	$ 0.0	$ (110.5)	$ (2,036.9)	$ 2,147.4	$ 0.0
Issuance of common stock under stock option and stock purchase plans (in shares)				0.2
Issuance of common stock under stock option and stock purchase plans	40.8			$ 0.0	40.8						40.8
Issuance of common stock under stock award plan (in shares)				0.4
Issuance of common stock under stock award plan	(53.8)			$ 0.0				(53.8)			(53.8)
Compensation expense related to share-based payments	191.2				191.2						191.2
Ending balance (in shares) at Dec. 31, 2019			0.0	198.0						23.8
Ending balance at Dec. 31, 2019	13,339.1		$ 0.0	$ 0.1	0.0	(135.2)		16,455.4		$ (2,977.1)	13,343.2		(4.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	4,060.5							4,000.6			4,000.6		59.9
Other comprehensive income (loss), net of tax	(162.8)					(163.8)					(163.8)		1.0
Capital contribution from noncontrolling interest	4.0										0.0		4.0
Distribution to noncontrolling interests	(75.0)												(75.0)
Repurchase of common stock pursuant to the share repurchase program, at cost (in shares)														(1.6)				(1.6)		(16.7)				(16.7)		(4.1)				(4.1)
Repurchase of common stock pursuant to the share repurchase program, at cost														$ (400.0)				$ (400.0)	$ (400.0)	$ (5,000.0)				$ (5,000.0)	$ (5,000.0)	$ (1,279.1)				$ (1,279.1)	(1,279.1)
Retirement of common stock pursuant to the share repurchase program, at cost (in shares)															1.6			1.6			16.7			16.7			4.1			4.1
Retirement of common stock pursuant to the share repurchase program, at cost														$ 0.0		$ (60.8)	$ (339.2)	$ 400.0		$ 0.0		$ (121.3)	$ (4,878.7)	$ 5,000.0	$ 0.0	$ 0.0		$ (71.0)	$ (1,208.1)	$ 1,279.1	$ 0.0
Issuance of common stock under stock option and stock purchase plans (in shares)				0.2
Issuance of common stock under stock option and stock purchase plans	49.3				49.3			0.0			49.3
Issuance of common stock under stock award plan (in shares)				0.4
Issuance of common stock under stock award plan	(53.7)				0.0			(53.7)			(53.7)
Compensation expense related to share-based payments	204.5				204.5						204.5
Other	(0.7)				(0.7)						(0.7)
Ending balance (in shares) at Dec. 31, 2020			0.0	176.2						23.8
Ending balance at Dec. 31, 2020	$ 10,686.1		$ 0.0	$ 0.1	$ 0.0	$ (299.0)		$ 13,976.3		$ (2,977.1)	$ 10,700.3		$ (14.2)