Collaboration profit (loss) sharing

No definition available.

Name:	biib_Collaborationprofitlosssharing
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Equity Method Investments, Expected Profit Share

+ References

No definition available.

+ Details

Name:	biib_EquityMethodInvestmentsExpectedProfitShare
Namespace Prefix:	biib_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Global Licensing Collaboration Agreement, Amount, Shares Purchased

+ References

No definition available.

+ Details

Name:	biib_GlobalLicensingCollaborationAgreementAmountSharesPurchased
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Global Licensing Collaboration Agreement, Purchase Price Per Share

+ References

No definition available.

+ Details

Name:	biib_GlobalLicensingCollaborationAgreementPurchasePricePerShare
Namespace Prefix:	biib_
Data Type:	dtr-types1:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Global Licensing Collaboration Agreement, Research, Development, Regulatory and Commercial Milestone Payments

+ References

No definition available.

+ Details

Name:	biib_GlobalLicensingCollaborationAgreementResearchDevelopmentRegulatoryAndCommercialMilestonePayments
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Global Licensing Collaboration Agreement, Shares, Purchased

+ References

No definition available.

+ Details

Name:	biib_GlobalLicensingCollaborationAgreementSharesPurchased
Namespace Prefix:	biib_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Share Of Net Profit From Sage Therapeutics, Percent

+ References

No definition available.

+ Details

Name:	biib_ShareOfNetProfitFromSageTherapeuticsPercent
Namespace Prefix:	biib_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 808
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479402/808-10-50-1

+ Details

Name:	us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of expense for research and development. Includes, but is not limited to, cost for computer software product to be sold, leased, or otherwise marketed and writeoff of research and development assets acquired in transaction other than business combination or joint venture formation or both. Excludes write-down of intangible asset acquired in business combination or from joint venture formation or both, used in research and development activity.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 48
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482785/280-10-55-48

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 985
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481283/985-20-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 730
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482916/730-10-50-1

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 912
-SubTopic 730
-Name Accounting Standards Codification
-Section 25
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479532/912-730-25-1

+ Details

Name:	us-gaap_ResearchAndDevelopmentExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of expense for research and development. Excludes cost for computer software product to be sold, leased, or otherwise marketed, writeoff of research and development assets acquired in transaction other than business combination or joint venture formation or both, and write-down of intangible asset acquired in business combination or from joint venture formation or both, used in research and development activity.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 730
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482916/730-10-50-1

+ Details

Name:	us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_TypeOfArrangementAxis=biib_SageTherapeuticsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=biib_DenaliTherapeuticsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=biib_SangamoTherapeuticsIncAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=biib_OtherresearchanddiscoveryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=biib_SamsungBiosimilarAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collaborative and Other Relationships - Other Arrangements (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended				3 Months Ended	12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Apr. 30, 2020	Dec. 31, 2019	Dec. 31, 2023	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Collaborative arrangements and non-collaborative arrangement transactions
Share of net profit from sage therapeutics, percent						50.00%
Research and development						$ 2,041.8	$ 2,462.0	$ 2,231.1
Sage Therapeutics
Collaborative arrangements and non-collaborative arrangement transactions
Global licensing collaboration, shares purchased, amount	$ 650.0
Global licensing collaboration, shares purchased (in shares)	6.2
Global licensing collaboration, purchase price per share (in dollars per share)	$ 104.14
Global licensing collaboration, development and commercial milestone payments	$ 700.0
Research and development expense					$ 75.0
Expected profit share percentage						50.00%
Collaboration profit (loss) sharing						$ 27.0	4.7
Denali Therapeutics
Collaborative arrangements and non-collaborative arrangement transactions
Global licensing collaboration, shares purchased, amount		$ 465.0
Global licensing collaboration, shares purchased (in shares)		13.0
Global licensing collaboration, purchase price per share (in dollars per share)		$ 34.94
Global licensing collaboration, development and commercial milestone payments		$ 1,100.0
Sangamo Therapeutics, Inc. Agreement
Collaborative arrangements and non-collaborative arrangement transactions
Global licensing collaboration, shares purchased, amount			$ 225.0
Global licensing collaboration, shares purchased (in shares)			24.0
Global licensing collaboration, purchase price per share (in dollars per share)			$ 9.21
Other research and discovery
Collaborative arrangements and non-collaborative arrangement transactions
Research and development						$ 54.0	$ 4.1	$ 39.2
Samsung Bioepis
Collaborative arrangements and non-collaborative arrangement transactions
Expected profit share percentage				45.00%		50.00%