Global Licensing Collaboration Agreement, Amount, Shares Purchased

No definition available.

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

- Definition

Global Licensing Collaboration Agreement, Payment, Achievement of First Commercial Sale Milestones

+ References

No definition available.

+ Details

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

- Definition

Global Licensing Collaboration Agreement, Payment, Achievement of Specific Sales Based Milestones

+ References

No definition available.

+ Details

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

- Definition

Global Licensing Collaboration Agreement, Payment, Selection of Targets

+ References

No definition available.

+ Details

Name:	biib_GlobalLicensingCollaborationAgreementPaymentSelectionOfTargets
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:	num: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

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 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

No definition available.

+ Details

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

- Definition

The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 985
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373

+ Details

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

- Definition

The costs incurred in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, excluding in-process research and development acquired in a business combination consummated during the period. Excludes software research and development, which has a separate concept.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373

+ 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:

Collaborative and Other Relationships - Other Arrangements (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended			12 Months Ended
	Nov. 30, 2020	Sep. 30, 2020	Feb. 29, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative arrangements and non-collaborative arrangement transactions
Research and development				$ 3,990.9	$ 2,280.6	$ 2,597.2
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
Research and development expense asset acquired	$ 209.0
Upfront and milestone payments made to collaborative partner	875.0
Global licensing collaboration, development and commercial milestone payments	$ 1,600.0
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
Research and development expense asset acquired		$ 41.3
Upfront and milestone payments made to collaborative partner		560.0
Global licensing collaboration, development and commercial milestone payments		$ 1,100.0
Research and development				8.8
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
Research and development expense asset acquired			$ 83.0
Upfront and milestone payments made to collaborative partner			125.0
Global licensing collaboration, development and commercial milestone payments			2,400.0
Research and development				$ 6.4
Term of collaboration agreement				5 years
Global licensing collaboration, payment, selection of targets			80.0
Global licensing collaboration, payment,first milestone			1,900.0
Global licensing collaboration, payment, sales based milestone			$ 380.0
Other research and discovery
Collaborative arrangements and non-collaborative arrangement transactions
Research and development				$ 92.1	$ 77.0	$ 48.6