Accrued expenses on funding commitment related to clinical research.

No definition available.

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

- Definition

Commitments And Contingencies.

+ References

No definition available.

+ Details

Name:	biib_CommitmentsAndContingenciesTextualAbstract
Namespace Prefix:	biib_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Cumulative sales level

+ References

No definition available.

+ Details

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

- Definition

Future contingent payment for annual worldwide net sales that exceed $2.0 billion

+ References

No definition available.

+ Details

Name:	biib_FutureContingentPaymentForAnnualWorldwideNetSalesThatExceed20Billion
Namespace Prefix:	biib_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Future contingent payment for annual worldwide net sales up to $2.0 billion

+ References

No definition available.

+ Details

Name:	biib_FutureContingentPaymentForAnnualWorldwideNetSalesUpTo20Billion
Namespace Prefix:	biib_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Future contingent payment threshold

+ References

No definition available.

+ Details

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

- Definition

Percentage of royalties as per collaboration.

+ References

No definition available.

+ Details

Name:	biib_PercentageOfRoyaltiesAsPerCollaboration
Namespace Prefix:	biib_
Data Type:	num: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

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

- Definition

For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

+ Details

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

- Definition

Amount of deferred tax liability attributable to taxable temporary differences from undistributed earnings of subsidiary and other recognized entity not within country of domicile. Includes, but is not limited to, other basis differences.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32621-109319

+ Details

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

- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

+ Details

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

- Definition

The amount of cash paid during the current period to foreign, federal, state, and local authorities as taxes on income.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4297-108586

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3536-108585

+ Details

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

- Definition

Amount recognized for uncertainty in income taxes classified as current.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=biib_TysabriProductMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	biib_FacilityLocationAxis=biib_SolothurnMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=biib_FumapharmAgMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	biib_CumulativeSalesLevelAxis=biib_TwentybillionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	biib_CumulativeSalesLevelAxis=biib_EachAdditionalOneBillionUpToTwentyBillionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	biib_FutureContingentMilestoneTypesAxis=biib_DevelopmentMilestonesMember
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:	biib_FutureContingentMilestoneTypesAxis=biib_CommercialMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeTaxAuthorityAxis=biib_U.S.2017TaxActMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=biib_IonisPharmaceuticalsMember
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_ProductOrServiceAxis=biib_SPINRAZAMember
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_AlkermesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

Commitments and Contingencies (Details Textual) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Apr. 02, 2013
Business Acquisition [Line Items]
Maximum contingent consideration in the form of development and approval milestones		$ 735.0
Increase to goodwill		117.5	$ 1,080.1
Potential future milestone payments commitment, approximately		6,800.0
Cancellable future commitments		514.0
Income Taxes Paid		87.0
Commitments And Contingencies (Textual)
Accrued expenses		24.0
Liabilities associated with uncertain tax positions		136.9
TYSABRI
Business Acquisition [Line Items]
Future contingent payment for annual worldwide net sales up to $2.0 billion					18.00%
Future contingent payment threshold					$ 2,000.0
Future contingent payment for annual worldwide net sales that exceed $2.0 billion					25.00%
Solothurn
Business Acquisition [Line Items]
Cancellable future commitments		52.0
TECFIDERA \| Fumapharm AG
Business Acquisition [Line Items]
Increase to goodwill	$ 300.0		1,200.0
Twenty billion \| TECFIDERA \| Fumapharm AG
Business Acquisition [Line Items]
Cumulative sales level	20,000.0		20,000.0
Increase to goodwill	300.0
Each additional one billion up to twenty billion \| TECFIDERA \| Fumapharm AG
Business Acquisition [Line Items]
Cumulative sales level	$ 20,000.0		$ 20,000.0
Development Milestones
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately		1,200.0
Regulatory Milestones
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately		1,400.0
Commercial Milestones [Member]
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately		4,200.0
2017 Tax Act
Business Acquisition [Line Items]
Transition toll tax liabilities		$ 697.0		$ 989.6
Ionis Pharmaceuticals \| Minimum \| SPINRAZA
Business Acquisition [Line Items]
Percentage Of Royalties As Per Collaboration		11.00%
Ionis Pharmaceuticals \| Maximum \| SPINRAZA
Business Acquisition [Line Items]
Percentage Of Royalties As Per Collaboration		15.00%
Alkermes
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately		$ 155.0
Alkermes \| VUMERITY
Business Acquisition [Line Items]
Percentage Of Royalties As Per Collaboration		15.00%
Twelve months
Business Acquisition [Line Items]
Potential future milestone payments commitment, approximately		$ 430.0