Consolidated Statements of Equity - USD ($) shares in Thousands, $ in Millions	Total	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, 2020		176,200
Beginning balance at Dec. 31, 2020	$ 10,686.1	$ 0.1	$ 0.0	$ (299.0)	$ 13,976.3	$ (2,977.1)	$ 10,700.3	$ (14.2)
Treasury stock at cost, beginning balance (in shares) at Dec. 31, 2020						23,800
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,727.6				1,556.1		1,556.1	171.5
Other comprehensive income (loss), net of tax	192.9			192.3			192.3	0.6
Distribution to noncontrolling interest	(100.0)							(100.0)
Capital contribution from noncontrolling interest	5.6							5.6
Repurchase of common stock pursuant to the share repurchase program, at cost (in shares)									(6,000)				(6,000)
Repurchase of common stock pursuant to the share repurchase program, at cost									$ (1,800.0)				$ (1,800.0)	$ (1,800.0)
Retirement of common stock pursuant to the share repurchase program, at cost (in shares)										6,000			6,000
Retirement of common stock pursuant to the share repurchase program, at cost									$ 0.0		$ (231.9)	$ (1,568.1)	$ 1,800.0
Issuance of common stock under stock option and stock purchase plans (in shares)		200
Issuance of common stock under stock option and stock purchase plans	54.4		54.4				54.4
Issuance of common stock under stock award plan (in shares)		400
Issuance of common stock under stock award plan	(55.0)		(2.4)		(52.6)		(55.0)
Compensation expense related to share-based payments	246.6		246.6				246.6
Other	$ 1.5		1.5				1.5
Common stock outstanding, ending balance (in shares) at Dec. 31, 2021	147,000	170,800
Ending balance at Dec. 31, 2021	$ 10,959.7	$ 0.1	68.2	(106.7)	13,911.7	$ (2,977.1)	10,896.2	63.5
Treasury stock at cost, ending balance (in shares) at Dec. 31, 2021						23,800
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)
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)				(3,600)
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			3,600
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
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
Issuance of common stock under stock award plan	(46.0)		(46.0)				(46.0)
Compensation expense 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	167,900
Ending balance at Dec. 31, 2022	$ 13,388.4	$ 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					23,800
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
Distribution to noncontrolling interest	(3.2)							(3.2)
Capital contribution from noncontrolling interest	$ 12.3							12.3
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
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
Issuance of common stock under stock award plan	(89.5)		(89.5)				(89.5)
Compensation expense 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	168,700
Ending balance at Dec. 31, 2023	$ 14,799.4	$ 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					23,800