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	Total Biogen Inc. shareholders’ equity	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
Common stock outstanding, beginning balance (in shares) at Dec. 31, 2021		0	170,800,000
Beginning balance at Dec. 31, 2021	$ 10,959.7	$ 0.0	$ 0.1	$ 68.2	$ (106.7)	$ 13,911.7	$ (2,977.1)	$ 10,896.2	$ 63.5
Treasury stock at cost, beginning balance (in shares) at Dec. 31, 2021							23,800,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	2,961.6					3,046.9		3,046.9	(85.3)
Other comprehensive income (loss), net of tax	(58.2)				(58.2)			(58.2)	0.0
Capital contribution from noncontrolling interest	12.3								12.3
Repurchase of common stock pursuant to the share repurchase program, at cost (in shares)										(3,600,000)				(3,600,000)
Repurchase of common stock pursuant to the share repurchase program, at cost										$ (750.0)				$ (750.0)	$ (750.0)
Retirement of common stock pursuant to the share repurchase program, at cost (in shares)											3,600,000			3,600,000
Retirement of common stock pursuant to the share repurchase program, at cost										$ 0.0		$ (257.9)	$ (492.1)	$ 750.0
Issuance of common stock under stock option and stock purchase plans (in shares)			200,000
Issuance of common stock under stock option and stock purchase plans	44.2			44.2				44.2
Issuance of common stock under stock award plan (in shares)			500,000
Issuance of common stock under stock award plan	(46.0)			(46.0)		0.0		(46.0)
Compensation related to share-based payments	263.5			263.5				263.5
Other	$ 1.3			1.3				1.3
Common stock outstanding, ending balance (in shares) at Dec. 31, 2022	144,000,000.0	0	167,900,000
Ending balance at Dec. 31, 2022	$ 13,388.4	$ 0.0	$ 0.1	73.3	(164.9)	16,466.5	$ (2,977.1)	13,397.9	(9.5)
Treasury stock at cost, ending balance (in shares) at Dec. 31, 2022							23,800,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,161.5					1,161.1		1,161.1	0.4
Other comprehensive income (loss), net of tax	11.2				11.2			11.2
Capital contribution from noncontrolling interest	12.3								12.3
Distribution to noncontrolling interest	$ (3.2)								(3.2)
Repurchase of common stock pursuant to the share repurchase program, at cost (in shares)	0
Issuance of common stock under stock option and stock purchase plans (in shares)			200,000
Issuance of common stock under stock option and stock purchase plans	$ 45.1			45.1				45.1
Issuance of common stock under stock award plan (in shares)			600,000
Issuance of common stock under stock award plan	(89.5)			(89.5)				(89.5)
Compensation related to share-based payments	274.4			274.4				274.4
Other	$ (0.8)			(0.8)				(0.8)
Common stock outstanding, ending balance (in shares) at Dec. 31, 2023	144,900,000	0	168,700,000
Ending balance at Dec. 31, 2023	$ 14,799.4	$ 0.0	$ 0.1	302.5	(153.7)	17,627.6	$ (2,977.1)	$ 14,799.4	$ 0.0
Treasury stock at cost, ending balance (in shares) at Dec. 31, 2023	23,800,000						23,800,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 1,632.2					1,632.2
Other comprehensive income (loss), net of tax	$ 17.5				17.5
Repurchase of common stock pursuant to the share repurchase program, at cost (in shares)	0
Issuance of common stock under stock option and stock purchase plans (in shares)			200,000
Issuance of common stock under stock option and stock purchase plans	$ 36.3			36.3
Issuance of common stock under stock award plan (in shares)			600,000
Issuance of common stock under stock award plan	(67.7)			(67.7)
Compensation related to share-based payments	301.5			301.5
Other	$ (3.2)			(3.2)
Common stock outstanding, ending balance (in shares) at Dec. 31, 2024	145,800,000	0	169,500,000
Ending balance at Dec. 31, 2024	$ 16,716.0	$ 0.0	$ 0.1	$ 569.4	$ (136.2)	$ 19,259.8	$ (2,977.1)
Treasury stock at cost, ending balance (in shares) at Dec. 31, 2024	23,800,000						23,800,000