Allocated research and development charge to the noncontrolling interest.

Name:	biib_AllocatedResearchAndDevelopmentChargeToNoncontrollingInterest
Namespace Prefix:	biib
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount associated with the Notice of Assessment of corporate tax including penalties and interest, subject to litigation.

+ Details

Name:	biib_AssessmentOfTaxIncludingPenaltiesAndInterest
Namespace Prefix:	biib
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Impact on effective tax rates of collaboration.

+ Details

Name:	biib_ImpactOnEffectiveTaxRatesOfCollaboration
Namespace Prefix:	biib
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Income Taxes.

+ Details

Name:	biib_IncomeTaxesTextualsAbstract
Namespace Prefix:	biib
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Proposed disallowance by tax authorities for payment for services.

+ Details

Name:	biib_ProposedDisallowanceByTaxAuthoritiesForPaymentForServices
Namespace Prefix:	biib
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Tax adjustments and settlements interest.

+ Details

Name:	biib_TaxAdjustmentsAndSettlementsInterest
Namespace Prefix:	biib
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

A ratio calculated by dividing the reported amount of income tax expense attributable to continuing operations for the period by GAAP-basis pretax income from continuing operations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(h)(2))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 109

-Paragraph 47

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph h

-Subparagraph 2

-Article 4

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 13

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319

+ Details

Name:	us-gaap_EffectiveIncomeTaxRateContinuingOperations
Namespace Prefix:	us-gaap
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the effects of adjustments of previously recorded tax expense, significant settlements of income tax disputes, and unusual tax positions or infrequent actions taken by the entity, including tax assessment reversal, IRS tax settlement and unusual repatriation of foreign earnings.

+ References

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

-Publisher FASB

-Name FASB Interpretation (FIN)

-Number 48

-Paragraph 10

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name FASB Interpretation (FIN)

-Number 48

-Paragraph 10A, 10B, 10C

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (a)(3)

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

+ Details

Name:	us-gaap_TaxAdjustmentsSettlementsAndUnusualProvisions
Namespace Prefix:	us-gaap
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.

+ Details

Name:	us-gaap_VariableInterestEntityLineItems
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Income Taxes (Details) (Textual) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 31, 2011	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007	Aug. 18, 2011	Jun. 08, 2010	Dec. 31, 2006
Variable Interest Entity [Line Items]
Impact on effective tax rates of collaboration			13.50%		2.70%
Income Taxes (Textuals) [Abstract]
Effective tax rate		26.40%	40.10%	26.00%	28.70%
Notice of Assessment of corporate excise tax including penalties and interest		$ 142.4		$ 142.4						$ 103.5	$ 38.9
Amount for settlement of exchange for payment of tax	7.0
Tax adjustments and settlement interest									5.0
Proposed disallowance by IRS for payment for services						130.0	130.0	130.0
Knopp [Member]
Variable Interest Entity [Line Items]
IPR&D charge allocated to noncontrolling interest			$ 145.0		$ 145.0