Collaborative Arrangement, Expected Development Cost Funds, Year One

No definition available.

Name:	biib_CollaborativeArrangementExpectedDevelopmentCostFundsYearOne
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Collaborative Arrangement, Expected Development Cost Funds, Year Two

+ References

No definition available.

+ Details

Name:	biib_CollaborativeArrangementExpectedDevelopmentCostFundsYearTwo
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Estimated additional payments upon achievement of development and commercial milestones.

+ References

No definition available.

+ Details

Name:	biib_EstimatedAdditionalPaymentsUponAchievementOfDevelopmentAndCommercialMilestones
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Reduction Of Research And Development Expense

+ References

No definition available.

+ Details

Name:	biib_ReductionOfResearchAndDevelopmentExpense
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Total upfront and milestone payments made to collaborative partner.

+ References

No definition available.

+ Details

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

- 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. 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

- Definition

Amount of revenue from collaborative arrangement. Excludes revenue from contract with customer under Topic 606.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-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

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 3
-SubTopic 10
-Topic 808
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479428/808-10-45-3

+ Details

Name:	us-gaap_RevenueFromCollaborativeArrangementExcludingRevenueFromContractWithCustomer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_ContingentConsiderationByTypeAxis=biib_MilestonePaymentFutureDevelopmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=biib_FelzartamabAndIzastobartHIB210Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAxis=biib_HumanImmunologyBiosciencesPreExistingInLicenseCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ContingentConsiderationByTypeAxis=biib_MilestonePaymentOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ContingentConsiderationByTypeAxis=biib_MilestonePaymentTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ContingentConsiderationByTypeAxis=biib_MilestonePaymentRegulatoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ContingentConsiderationByTypeAxis=biib_MilestonePaymentCommercialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_TypeOfArrangementAxis=biib_AlteogenInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=biib_AlloyTherapeuticsInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collaborative and Other Relationships - Other Research and Discovery Arrangements and Funding Arrangements (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 31, 2026	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Jun. 30, 2026	Dec. 31, 2025
Collaborative arrangements and non-collaborative arrangement transactions
Research and development:		$ 574.0	$ 436.1	$ 1,642.6	$ 1,269.2
Reduction of research and development expense		0.0	50.0		150.0
Milestone Payment, Future Development
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones		250.0		250.0
Felzartamab And Izastobart/HIB210 \| Human Immunology Biosciences Pre-Existing In License Commitments \| Milestone Payment, One
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones		35.0		35.0
Felzartamab And Izastobart/HIB210 \| Human Immunology Biosciences Pre-Existing In License Commitments \| Milestone Payment, Two
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones		30.0		30.0
Felzartamab And Izastobart/HIB210 \| Human Immunology Biosciences Pre-Existing In License Commitments \| Milestone Payment, Future Development
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones		130.0		130.0
Felzartamab And Izastobart/HIB210 \| Human Immunology Biosciences Pre-Existing In License Commitments \| Milestone Payment, Regulatory
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones		230.0		230.0
Felzartamab And Izastobart/HIB210 \| Human Immunology Biosciences Pre-Existing In License Commitments \| Milestone Payment, Commercial
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones		640.0		640.0
Collaborative Arrangement, Transaction with Party to Collaborative Arrangement
Collaborative arrangements and non-collaborative arrangement transactions
Expected receipts to co-fund development costs, current							$ 200.0
Expected receipts to co-fund development costs, noncurrent		50.0		50.0
Other research and discovery
Collaborative arrangements and non-collaborative arrangement transactions
Research and development:		51.3	$ 2.1	57.3	$ 19.4
Sobi
Collaborative arrangements and non-collaborative arrangement transactions
Revenue recognized		$ 18.4		$ 31.8
Alteogen Inc.
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones						$ 10.0
Upfront and milestone paymentsmade to collaborative partner	$ 20.0
Alloy Therapeutics, Inc.
Collaborative arrangements and non-collaborative arrangement transactions
Upfront and milestone paymentsmade to collaborative partner	$ 12.0