Additional milestone payment.

No definition available.

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

- 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

Estimated Additional Payments Upon Initiation Of New Phases

+ References

No definition available.

+ Details

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

- Definition

Expected additional milestone payments when meeting certain sales threshold.

+ References

No definition available.

+ Details

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

- Definition

Investment In Common Stock, Shares Purchased

+ References

No definition available.

+ Details

Name:	biib_InvestmentInCommonStockSharesPurchased
Namespace Prefix:	biib_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of royalties as per collaboration.

+ References

No definition available.

+ Details

Name:	biib_PercentageOfRoyaltiesAsPerCollaboration
Namespace Prefix:	biib_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Purchase of common stock

+ References

No definition available.

+ Details

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

- Definition

Research And Development, Gene Targets

+ References

No definition available.

+ Details

Name:	biib_ResearchAndDevelopmentGeneTargets
Namespace Prefix:	biib_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Term Of Collaboration Agreement

+ References

No definition available.

+ Details

Name:	biib_TermOfCollaborationAgreement
Namespace Prefix:	biib_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Total payment to enter collaboration agreement

+ References

No definition available.

+ Details

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

- Definition

Upfront Payment For Collaboration Agreement

+ References

No definition available.

+ Details

Name:	biib_UpfrontPaymentForCollaborationAgreement
Namespace Prefix:	biib_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4A
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476153/220-40-55-11

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476153/220-40-55-4

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(2)(d))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(2)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 924
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 11.L)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479941/924-10-S99-1

Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476148/220-40-50-6

+ Details

Name:	us-gaap_CostOfGoodsAndServicesSold
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_IonisPharmaceuticalsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_ProductOrServiceAxis=biib_SOD1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Collaborative and Other Relationships - Ionis (Details) shares in Millions, $ in Millions	1 Months Ended					12 Months Ended
	Apr. 30, 2023	Dec. 31, 2019 USD ($)	Jun. 30, 2018 USD ($) shares	Sep. 30, 2013	Dec. 31, 2012 geneTarget	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2017 USD ($)
Collaborative arrangements and non-collaborative arrangement transactions
Cost of sales, excluding amortization and impairment of acquired intangible assets						$ 2,404.2	$ 2,310.4	$ 2,533.4
Research and development						1,778.6	1,980.3	2,445.4
Ionis Pharmaceuticals
Collaborative arrangements and non-collaborative arrangement transactions
Term of collaboration agreement			10 years	6 years
Total payment to enter collaboration agreement			$ 1,000.0
Upfront payment for collaboration agreement			$ 375.0
Investment in common stock, shares purchased (in shares) \| shares			11.5
Purchase of common stock			$ 625.0
Estimated additional payments upon achievement of development and commercial milestones									$ 155.0
Research and development						7.5	7.5	7.5
Expected additional milestone payments when certain sales threshold is met									$ 400.0
Gene targets \| geneTarget					3
Additional milestone payment		$ 155.0
Ionis Pharmaceuticals \| SPINRAZA
Collaborative arrangements and non-collaborative arrangement transactions
Cost of sales, excluding amortization and impairment of acquired intangible assets						$ 212.3	216.1	240.2
Ionis Pharmaceuticals \| SOD1
Collaborative arrangements and non-collaborative arrangement transactions
Research and development								$ 16.0
Ionis Pharmaceuticals \| QALSODY
Collaborative arrangements and non-collaborative arrangement transactions
Research and development							20.0
Ionis Pharmaceuticals \| QALSODY Japan
Collaborative arrangements and non-collaborative arrangement transactions
Research and development							$ 10.0
Ionis Pharmaceuticals \| Minimum
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones			125.0
Ionis Pharmaceuticals \| Minimum \| SPINRAZA
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of royalties as per collaboration						11.00%
Ionis Pharmaceuticals \| Minimum \| QALSODY
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of royalties as per collaboration	11.00%
Ionis Pharmaceuticals \| Maximum
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon achievement of development and commercial milestones			$ 270.0
Ionis Pharmaceuticals \| Maximum \| SPINRAZA
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of royalties as per collaboration						15.00%
Ionis Pharmaceuticals \| Maximum \| QALSODY
Collaborative arrangements and non-collaborative arrangement transactions
Percentage of royalties as per collaboration	15.00%
Ionis Pharmaceuticals Inc.
Collaborative arrangements and non-collaborative arrangement transactions
Estimated additional payments upon initiation of new phases						$ 45.0