Condensed Consolidated Statement of Equity Statement - USD ($) shares in Millions, $ in Millions	Total	Preferred stock	Common stock	Additional paid-in capital	Accumulated Other Comprehensive Income	Retained earnings	Treasury stock	Noncontrolling interest	Parent	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 Parent	2019 Share Repurchase Program	2019 Share Repurchase Program Common stock	2019 Share Repurchase Program Additional paid-in capital	2019 Share Repurchase Program Retained earnings	2019 Share Repurchase Program Treasury stock	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
Beginning balance at Dec. 31, 2018	$ 13,031.6	$ 0.0	$ 0.1	$ 0.0	$ (240.4)	$ 16,257.0	$ (2,977.1)
Beginning balance, shares at Dec. 31, 2018		0.0	(221.0)				(23.8)
Beginning balance at Dec. 31, 2018									$ 13,039.6
Beginning balance at Dec. 31, 2018								$ (8.0)
Net income attributable to Biogen Inc.	1,408.8					1,408.8			1,408.8
Net income (loss) attributable to noncontrolling interest, net of tax	0.0							0.0
Net income	1,408.8
Other comprehensive income (loss), net of tax	20.6				20.6				20.6
Other comprehensive income (loss), net of tax	20.6							0.0
Noncontrolling interest, increase (decrease) other	0.1							0.1	0.0
Repurchase of common stock, at cost										$ (655.8)				$ (655.8)	$ (655.8)
Repurchase of common stock, at cost, shares														(2.4)
Retirement of common stock pursuant to Share Repurchase Progams, at cost										$ 0.0	$ 0.0	$ (65.6)	$ (590.2)	$ (655.8)	$ 0.0
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares											(2.4)			(2.4)
Issuance of common stock under stock option and stock purchase plans	16.6		$ 0.0	16.6					16.6
Issuance of common stock under stock option and stock purchase plans, shares			0.1
Issuance of common stock under award plan			0.3
Issuance of common stock under stock award plan	48.9		$ 0.0	0.0					(48.9)				$ (48.9)
Compensation related to share-based payments	(49.0)			(49.0)		0.0			(49.0)
Ending balance at Mar. 31, 2019	13,822.0	$ 0.0	$ 0.1	0.0	(219.8)	17,026.7	$ (2,977.1)
Ending balance at Mar. 31, 2019									$ 13,829.9
Ending balance at Mar. 31, 2019								(7.9)
Ending balance, shares at Mar. 31, 2019		0.0	(219.0)				(23.8)
Beginning balance at Dec. 31, 2019	13,339.1	$ 0.0	$ 0.1	0.0	(135.2)	16,455.4	$ (2,977.1)
Beginning balance, shares at Dec. 31, 2019		0.0	(198.0)				(23.8)
Beginning balance at Dec. 31, 2019	13,343.2
Beginning balance at Dec. 31, 2019	(4.1)							(4.1)
Net income attributable to Biogen Inc.	1,399.1					1,399.1
Net income (loss) attributable to noncontrolling interest, net of tax	(6.5)							(6.5)
Net income	1,392.6
Other comprehensive income (loss), net of tax	(14.1)				(14.1)
Other comprehensive income (loss), net of tax	(13.5)							0.6
Repurchase of common stock, at cost																$ (1,279.1)				$ (1,279.1)	$ (941.1)				$ (941.1)
Repurchase of common stock, at cost, shares																(4.1)				(4.1)	(3.2)				(3.2)
Retirement of common stock pursuant to Share Repurchase Progams, at cost																$ 0.0	$ 0.0	$ (71.0)	$ (1,208.1)	$ 1,279.1	$ 0.0	$ 0.0	$ (15.6)	$ (925.5)	$ 941.1
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares																	(4.1)			(4.1)		(3.2)			(3.2)
Issuance of common stock under stock option and stock purchase plans	18.1		$ 0.0	18.1
Issuance of common stock under stock option and stock purchase plans, shares			0.1
Issuance of common stock under award plan			0.3
Issuance of common stock under stock award plan	(47.8)		$ 0.0	0.0		47.8
Compensation related to share-based payments	69.2			69.2
Adoption of new accounting guidance	(0.6)			(0.6)
Ending balance at Mar. 31, 2020	12,536.9	$ 0.0	$ 0.1	$ 0.1	$ (149.3)	$ 15,673.1	$ (2,977.1)
Ending balance at Mar. 31, 2020	12,546.9
Ending balance at Mar. 31, 2020	$ (10.0)							$ (10.0)
Ending balance, shares at Mar. 31, 2020		0.0	(191.1)				(23.8)