Consolidated Statements of Equity - USD ($) shares in Millions, $ in Millions	Total	Total Biogen Inc. shareholders’ equity	Preferred stock	Common stock	Additional paid-in capital	Accumulated other comprehensive income (loss)	Retained earnings	Treasury stock	Noncontrolling interests
Common stock outstanding, beginning balance (in shares) at Dec. 31, 2022			0.0	167.9
Beginning balance at Dec. 31, 2022	$ 13,388.4	$ 13,397.9	$ 0.0	$ 0.1	$ 73.3	$ (164.9)	$ 16,466.5	$ (2,977.1)	$ (9.5)
Treasury stock at cost, beginning balance (in shares) at Dec. 31, 2022								23.8
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
Deconsolidation of noncontrolling interest	(3.2)								(3.2)
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	45.1	45.1			45.1
Issuance of common stock under stock award plan (in shares)				0.6
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	144.9		0.0	168.7
Ending balance at Dec. 31, 2023	$ 14,799.4	$ 14,799.4	$ 0.0	$ 0.1	302.5	(153.7)	17,627.6	$ (2,977.1)	$ 0.0
Treasury stock at cost, ending balance (in shares) at Dec. 31, 2023								23.8
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
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	36.3				36.3
Issuance of common stock under stock award plan (in shares)				0.6
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	145.8		0.0	169.5
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	23.8							23.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 1,292.9						1,292.9
Other comprehensive income (loss), net of tax	(45.8)					(45.8)
Issuance of common stock under stock option and stock purchase plans (in shares)				0.3
Issuance of common stock under stock option and stock purchase plans	34.0				34.0
Issuance of common stock under stock award plan (in shares)				0.7
Issuance of common stock under stock award plan	(44.0)				(44.0)
Compensation related to share-based payments	303.1				303.1
Other	$ 0.6				0.6
Common stock outstanding, ending balance (in shares) at Dec. 31, 2025	146.8		0.0	170.5
Ending balance at Dec. 31, 2025	$ 18,256.8		$ 0.0	$ 0.1	$ 863.1	$ (182.0)	$ 20,552.7	$ (2,977.1)
Treasury stock at cost, ending balance (in shares) at Dec. 31, 2025	23.8							23.8