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	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	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	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	Accounting Standards Update 2016-01 Accumulated Other Comprehensive Income	Accounting Standards Update 2016-16 Retained earnings
Beginning balance at Dec. 31, 2017	$ 12,598.1	$ 0.0	$ 0.1	$ 97.8	$ (318.4)	$ 15,810.4	$ (2,977.1)
Beginning balance, shares at Dec. 31, 2017		0.0	(235.3)				(23.8)
Beginning balance at Dec. 31, 2017	12,612.8
Beginning balance at Dec. 31, 2017								$ (14.7)
Net income attributable to Biogen Inc.	3,483.9					3,483.9
Net income (loss) attributable to noncontrolling interest, net of tax	45.2							45.2
Net income	3,529.1
Other comprehensive income (loss), net of tax	68.6				68.6
Other comprehensive income (loss), net of tax	68.3							(0.3)
Noncontrolling interest, increase (decrease) other	13.0							13.0	$ 0.0
Noncontrolling interest, decrease from distributions to noncontrolling interest holders	(50.0)							50.0	0.0
Repurchase of common stock, at cost																				$ (3,000.0)				$ (3,000.0)
Repurchase of common stock, at cost, shares																				(10.5)				(10.5)
Retirement of common stock pursuant to Share Repurchase Progams, at cost																				$ 0.0	$ 0.0	$ (171.2)	$ (2,828.8)	$ (3,000.0)
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares																					(10.5)			(10.5)
Issuance of common stock under stock option and stock purchase plans	33.3		$ 0.0	33.3
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	(39.9)		$ 0.0	(39.9)
Compensation related to share-based payments	128.0			127.8		0.2
Adoption of new accounting guidance	479.9																								$ 1.5	$ 478.4
Ending balance at Sep. 30, 2018	13,759.8	$ 0.0	$ 0.1	47.8	(248.3)	16,944.1	$ (2,977.1)
Ending balance at Sep. 30, 2018	13,766.6
Ending balance at Sep. 30, 2018								(6.8)
Ending balance, shares at Sep. 30, 2018		0.0	(225.2)				(23.8)
Beginning balance at Jun. 30, 2018	12,253.7	$ 0.0	$ 0.1	0.0	(261.6)	15,499.5	$ (2,977.1)
Beginning balance, shares at Jun. 30, 2018		0.0	(225.2)				(23.8)
Beginning balance at Jun. 30, 2018	12,260.9
Beginning balance at Jun. 30, 2018								(7.2)
Net income attributable to Biogen Inc.	1,444.4					1,444.4
Net income (loss) attributable to noncontrolling interest, net of tax	(1.5)							(1.5)
Net income	1,442.9
Other comprehensive income (loss), net of tax	13.3				13.3
Other comprehensive income (loss), net of tax	13.3							0.0
Noncontrolling interest, increase (decrease) other	1.9							1.9	0.0
Issuance of common stock under stock option and stock purchase plans	8.7		$ 0.0	8.7
Issuance of common stock under stock option and stock purchase plans, shares			0.0
Issuance of common stock under award plan			0.0
Issuance of common stock under stock award plan	(1.1)		$ 0.0	(1.1)
Compensation related to share-based payments	40.4			40.2		0.2
Ending balance at Sep. 30, 2018	13,759.8	$ 0.0	$ 0.1	47.8	(248.3)	16,944.1	$ (2,977.1)
Ending balance at Sep. 30, 2018	13,766.6
Ending balance at Sep. 30, 2018								(6.8)
Ending balance, shares at Sep. 30, 2018		0.0	(225.2)				(23.8)
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)							(8.0)
Net income attributable to Biogen Inc.	4,448.8					4,448.8
Net income (loss) attributable to noncontrolling interest, net of tax	0.0							0.0
Net income	4,448.8
Other comprehensive income (loss), net of tax	148.1				148.1
Other comprehensive income (loss), net of tax	147.7							(0.4)
Noncontrolling interest, increase (decrease) other	4.3							4.3	0.0
Repurchase of common stock, at cost										$ (1,627.8)				$ (1,627.8)	$ (2,147.4)				$ (2,147.4)
Repurchase of common stock, at cost, shares										(7.0)				(7.0)	(8.9)				(8.9)
Retirement of common stock pursuant to Share Repurchase Progams, at cost										$ 0.0	$ 0.0	$ (74.8)	$ (1,553.0)	$ (1,627.8)	$ 0.0	$ 0.0	$ (110.5)	$ (2,036.9)	$ (2,147.4)
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares											(7.0)			(7.0)		(8.9)			(8.9)
Issuance of common stock under stock option and stock purchase plans	33.5		$ 0.0	33.5
Issuance of common stock under stock option and stock purchase plans, shares			0.2
Issuance of common stock under award plan			0.4
Issuance of common stock under stock award plan	(50.7)		$ 0.0	0.0		(50.7)
Compensation related to share-based payments	151.8			151.8
Ending balance at Sep. 30, 2019	13,991.8	$ 0.0	$ 0.1	0.0	(92.3)	17,065.2	$ (2,977.1)
Ending balance at Sep. 30, 2019	13,995.9
Ending balance at Sep. 30, 2019	(4.1)							(4.1)
Ending balance, shares at Sep. 30, 2019		0.0	(205.7)				(23.8)
Beginning balance at Jun. 30, 2019	12,948.8	$ 0.0	$ 0.1	0.0	(252.9)	16,182.8	$ (2,977.1)
Beginning balance, shares at Jun. 30, 2019		0.0	(208.6)				(23.8)
Beginning balance at Jun. 30, 2019	12,952.9
Beginning balance at Jun. 30, 2019								(4.1)
Net income attributable to Biogen Inc.	1,545.9					1,545.9
Net income (loss) attributable to noncontrolling interest, net of tax	0.0							0.0
Net income	1,545.9
Other comprehensive income (loss), net of tax	160.6				160.6
Other comprehensive income (loss), net of tax	160.6							0.0
Noncontrolling interest, increase (decrease) other	0.0							0.0	$ 0.0
Repurchase of common stock, at cost										$ (717.9)				$ (717.9)
Repurchase of common stock, at cost, shares										(3.1)				(3.1)
Retirement of common stock pursuant to Share Repurchase Progams, at cost										$ 0.0	$ 0.0	$ (55.1)	$ (662.8)	$ (717.9)
Retirement of common stock pursuant to Share Repurchase Programs, at cost, shares											(3.1)			(3.1)
Issuance of common stock under stock option and stock purchase plans	7.3		$ 0.0	7.3
Issuance of common stock under stock option and stock purchase plans, shares			0.1
Issuance of common stock under award plan			0.1
Issuance of common stock under stock award plan	(0.7)		$ 0.0	0.0		(0.7)
Compensation related to share-based payments	47.8			47.8
Ending balance at Sep. 30, 2019	13,991.8	$ 0.0	$ 0.1	$ 0.0	$ (92.3)	$ 17,065.2	$ (2,977.1)
Ending balance at Sep. 30, 2019	13,995.9
Ending balance at Sep. 30, 2019	$ (4.1)							$ (4.1)
Ending balance, shares at Sep. 30, 2019		0.0	(205.7)				(23.8)