Leases (Details Textual) - USD ($) $ in Millions	Sep. 30, 2019	Aug. 01, 2019
Minimum
Operating Leased Assets [Line Items]
Lessee, Operating Lease, Term of Contract	1 year
Lessee, Operating Lease, Renewal Term	1 year
Maximum
Operating Leased Assets [Line Items]
Lessee, Operating Lease, Term of Contract	9 years
Lessee, Operating Lease, Renewal Term	6 years
Denmark Manufacturing Operations
Operating Leased Assets [Line Items]
Operating lease assets		$ 2.2
Operating lease liabilities		$ 2.2

Disposal Group, Operating Lease Assets

No definition available.

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

- Definition

Disposal Group, Operating lease liabilities

+ References

No definition available.

+ Details

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

- Definition

Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseRenewalTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_LesseeOperatingLeaseTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
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

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
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- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
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- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=biib_DenmarkManufacturingOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
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