Acquisitions -Narrative (Details)			12 Months Ended
	Jul. 02, 2024 USD ($)	Sep. 26, 2023 USD ($) ft² $ / shares	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Sep. 27, 2023 USD ($)
Business Acquisition [Line Items]
Share-based compensation expense			$ (301,500,000)	$ (274,400,000)	$ (263,500,000)
Contingent consideration obligations			291,200,000
Contingent consideration obligations			221,600,000
Goodwill(1)			6,478,900,000	6,219,200,000	5,749,000,000
Selling, general and administrative			2,403,700,000	2,549,700,000	2,403,600,000
Long-term debt, gross			6,567,300,000
Net Income attributable to Biogen Inc.			1,632,200,000	1,161,500,000	2,961,600,000
Cost of sales, excluding amortization and impairment of acquired intangible assets			$ 2,310,400,000	2,533,400,000	2,278,300,000
Milestone Payment, Future Development | Felzartamab And Izastobart/HIB210 | Human Immunology Biosciences Pre-Existing In License Commitments
Business Acquisition [Line Items]
Contingent consideration obligations	$ 130,000,000
Milestone Payment, Regulatory | Felzartamab And Izastobart/HIB210 | Human Immunology Biosciences Pre-Existing In License Commitments
Business Acquisition [Line Items]
Contingent consideration obligations	230,000,000
Milestone Payment, Commercial | Felzartamab And Izastobart/HIB210 | Human Immunology Biosciences Pre-Existing In License Commitments
Business Acquisition [Line Items]
Contingent consideration obligations	640,000,000
Minimum
Business Acquisition [Line Items]
Operating lease, contract term			1 year
Maximum
Business Acquisition [Line Items]
Operating lease, contract term			14 years
Research and development
Business Acquisition [Line Items]
Share-based compensation expense			$ 154,100,000	296,700,000	98,500,000
Selling, general and administrative
Business Acquisition [Line Items]
Share-based compensation expense			198,600,000	371,700,000	175,100,000
Human Immunology Biosciences
Business Acquisition [Line Items]
Total transaction value	1,137,300,000
Range of outcomes, value, high	650,000,000
Non-vested equity awards	74,500,000
Share-based compensation expense	56,400,000		56,400,000	0	$ 0
Working capital adjustment	43,700,000
Contingent consideration obligations	485,100,000
Contingent consideration obligations	279,300,000
Contingent consideration obligations	205,800,000
Adjustment, consideration transferred			0
Intangible assets:	1,600,000,000
Goodwill(1)	256,500,000
Acquisition related costs			$ 2,800,000
Operating lease liabilities	1,200,000
Operating lease assets	1,200,000
Human Immunology Biosciences | Felzartamab (IgAN)
Business Acquisition [Line Items]
Intangible assets:	920,000,000
Human Immunology Biosciences | Felzartamab (AMR)
Business Acquisition [Line Items]
Intangible assets:	450,000,000
Human Immunology Biosciences | Felzartamab (PMN)
Business Acquisition [Line Items]
Intangible assets:	$ 265,000,000
Human Immunology Biosciences | Risk-adjusted discount rate
Business Acquisition [Line Items]
Measurement input	0.145
Human Immunology Biosciences | Measurement Input, Probability Rate
Business Acquisition [Line Items]
Discount rate	0.670		0.683
Human Immunology Biosciences | Minimum | Risk-adjusted discount rate
Business Acquisition [Line Items]
Discount rate	0.062
Human Immunology Biosciences | Maximum | Phase 3 Clinical Trial, Felzartamab One
Business Acquisition [Line Items]
Contingent consideration obligations	$ 150,000,000
Human Immunology Biosciences | Maximum | Phase 3 Clinical Trial, Felzartamab Two
Business Acquisition [Line Items]
Contingent consideration obligations	$ 150,000,000
Human Immunology Biosciences | Maximum | Risk-adjusted discount rate
Business Acquisition [Line Items]
Discount rate	0.070
Human Immunology Biosciences | Previously Reported
Business Acquisition [Line Items]
Total transaction value	$ 1,150,000,000
Human Immunology Biosciences | Research and development
Business Acquisition [Line Items]
Share-based compensation expense	42,500,000
Human Immunology Biosciences | Selling, general and administrative
Business Acquisition [Line Items]
Share-based compensation expense	13,900,000
Human Immunology Biosciences | In-process research and development (omaveloxolone) | Felzartamab (IgAN)
Business Acquisition [Line Items]
Intangible assets:	920,000,000.0
Human Immunology Biosciences | In-process research and development (omaveloxolone) | Felzartamab (AMR)
Business Acquisition [Line Items]
Intangible assets:	450,000,000.0
Human Immunology Biosciences | In-process research and development (omaveloxolone) | Felzartamab (PMN)
Business Acquisition [Line Items]
Intangible assets:	265,000,000.0
Human Immunology Biosciences | Other clinical programs
Business Acquisition [Line Items]
Intangible assets:	$ 7,900,000
Reata Pharmaceuticals, Inc
Business Acquisition [Line Items]
Total transaction value		$ 6,602,900,000
Goodwill(1)			$ 469,200,000			$ 469,200,000
Price per share (in dollars per share) | $ / shares		$ 172.50
Outstanding equity awards			983,900,000
Consideration transferred, equity compensation pre-acquisition services		$ 590,500,000
Operating expenses		393,400,000
Selling, general and administrative		196,400,000
Research and development expense		197,000,000
Inventory		$ 1,300,000,000	1,259,000,000
Inventory amortization period		4 years
Fair value indefinite lived intangible assets discount rate		14.30%
Operating lease area | ft²		327,400
Operating lease, contract term		16 years
Operating lease liabilities			151,800,000			151,800,000
Remaining lease term		15 years
Operating lease assets			121,200,000			121,200,000
Reata Pharmaceuticals, Inc | Fair Value Adjustment to Inventory
Business Acquisition [Line Items]
Net Income attributable to Biogen Inc.			230,000,000.0	31,500,000
Reata Pharmaceuticals, Inc | Clinical Inventory | Fair Value Adjustment to Inventory
Business Acquisition [Line Items]
Cost of sales, excluding amortization and impairment of acquired intangible assets			48,500,000
Reata Pharmaceuticals, Inc | Secured Debt | The Credit Facility | Line of Credit
Business Acquisition [Line Items]
Long-term debt, gross		$ 1,000,000,000
Reata Pharmaceuticals, Inc | Selling, general and administrative
Business Acquisition [Line Items]
Separately recognized transactions, additional disclosures, acquisition cost expensed				$ 28,400,000
Reata Pharmaceuticals, Inc | Completed technology for SKYCLARYS (U.S.)
Business Acquisition [Line Items]
Intangible assets:		4,200,000,000	4,200,000,000			4,200,000,000
Reata Pharmaceuticals, Inc | In-process research and development (omaveloxolone)
Business Acquisition [Line Items]
Intangible assets:		2,300,000,000	2,300,000,000			2,300,000,000
Reata Pharmaceuticals, Inc | Priority review voucher
Business Acquisition [Line Items]
Intangible assets:		$ 100,000,000	100,000,000.0
Reata Pharmaceuticals, Inc | Other clinical programs
Business Acquisition [Line Items]
Intangible assets:			$ 40,000,000.0			$ 40,000,000