Additional reduction in royalty rate payable on commercial sales

No definition available.

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

- Definition

Investment in biotechnology companies that are determined to be unconsolidated variable interest entities.

+ References

No definition available.

+ Details

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

- Definition

Percentage Of Future Development Costs

+ References

No definition available.

+ Details

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

- Definition

Potential future milestone payments commitment to third party approximately.

+ References

No definition available.

+ Details

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

- Definition

Reduction in royalty rate payable on commercial sales

+ References

No definition available.

+ Details

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

- Definition

Research and Development Costs, Percentage

+ References

No definition available.

+ Details

Name:	biib_ResearchAndDevelopmentCostsPercentage
Namespace Prefix:	biib_
Data Type:	dtr-types:percentItemType
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

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121826272&loc=d3e32537-109319

+ Details

Name:	us-gaap_DeferredTaxAssetsGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, after allocation of valuation allowances and deferred tax liability, of deferred tax asset attributable to deductible differences and carryforwards, without jurisdictional netting.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121826272&loc=d3e32537-109319

+ Details

Name:	us-gaap_DeferredTaxAssetsLiabilitiesNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121826272&loc=d3e32537-109319

+ Details

Name:	us-gaap_DeferredTaxAssetsValuationAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of income (loss) for proportionate share of equity method investee's income (loss).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(12))
-URI http://asc.fasb.org/extlink&oid=123367319&loc=SL114868664-224227

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(10))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13)(f))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=123359005&loc=d3e8736-108599

Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570

+ Details

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

- Definition

Amount of cash outflow to a noncontrolling interest. Includes, but not limited to, reduction of noncontrolling interest ownership. Excludes dividends paid to the noncontrolling interest.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3291-108585

+ Details

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

- Definition

Amount of increase (decrease) in unrecognized tax benefits attributable to uncertain tax positions taken in tax returns.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121826272&loc=SL6600010-109319

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
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_VariableInterestEntityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

Name:	srt_ConsolidatedEntitiesAxis=biib_NeurimmuneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	biib_FutureContingentMilestoneTypesAxis=biib_RegulatoryMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	biib_ExpectedPaymentTimelineofMilestoneAxis=biib_TwelvemonthsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=us-gaap_EuropeanUnionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=country_JP
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Investments in Variable Interest Entities (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended			12 Months Ended
	May 31, 2018	Oct. 31, 2017	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately						$ 10,000.0
Income (loss) from equity method investments						34.9	$ 3.3	$ (77.4)
Deferred tax asset, net			$ 490.0
Deferred tax asset, gross			945.0
Increase in unrecognized tax benefit			455.0
Valuation allowance						1,961.3	1,753.9
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities						24.6	12.8
ADUHELM
Variable Interest Entity [Line Items]
Valuation allowance						$ 390.0
Neurimmune
Variable Interest Entity [Line Items]
Term of collaboration agreement						12 years
Research and development costs, percentage						100.00%
Payment to Neurimmune	$ 50.0	$ 150.0
Reduction in royalty rate payable on commercial sales		15.00%
Additional reduction in royalty rate payable on commercial sales	5.00%
Regulatory Milestones
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately						$ 900.0
Regulatory Milestones \| Neurimmune
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately				$ 75.0
Eisai
Variable Interest Entity [Line Items]
Income (loss) from equity method investments				$ 33.8	$ 33.8	$ 45.0	$ 33.8	$ 0.0
Percentage of future development costs						45.00%
Eisai \| ADUHELM
Variable Interest Entity [Line Items]
Income (loss) from equity method investments			45.0			$ 45.0
Eisai \| Twelve months \| ADUHELM
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately						100.0
Eisai \| Regulatory Milestones \| European Union \| ADUHELM
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately						50.0
Eisai \| Regulatory Milestones \| JAPAN \| ADUHELM
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately						$ 50.0
Eisai \| Regulatory Milestones \| Neurimmune \| ADUHELM
Variable Interest Entity [Line Items]
Potential future milestone payments commitment to third party approximately			$ 100.0