Proposed disallowance by tax authorities for payment for services.

No definition available.

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

- Definition

The amount of estimated penalties and interest accrued as of the balance sheet date arising from income tax examinations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319

+ Details

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

- Definition

Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 15A

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Unrecognized Tax Benefit

-URI http://asc.fasb.org/extlink&oid=6527854

+ Details

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

- Details

Name:	us-gaap_TaxPeriodAxis=us-gaap_TaxYear2007Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TaxPeriodAxis=us-gaap_TaxYear2008Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TaxPeriodAxis=us-gaap_TaxYear2009Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Income Taxes (Details Textual) - USD ($) $ in Millions	1 Months Ended
Income Taxes (Details Textual) - USD ($) $ in Millions	Oct. 31, 2011	Sep. 30, 2015	Mar. 31, 2015
Income Tax Contingency [Line Items]
Notice of assessment of corporate withholding tax including penalties and interest			$ 49.3
Net benefit for a previously unrecognized position		$ 24.0
Tax Year 2007
Income Tax Contingency [Line Items]
Proposed deduction disallowance by tax authorities	$ 130.0
Tax Year 2008
Income Tax Contingency [Line Items]
Proposed deduction disallowance by tax authorities	130.0
Tax Year 2009
Income Tax Contingency [Line Items]
Proposed deduction disallowance by tax authorities	$ 130.0