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Income Taxes (Details Textual) (USD $) In Millions, unless otherwise specified	3 Months Ended		1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Oct. 31, 2011
Income Tax Contingency [Line Items]
Effective tax rate	25.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations	26.80%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Notice of assessment of corporate withholding tax including penalties and interest	$ 45.6us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued
Net benefit for a previously unrecognized position	16.4biib_NetBenefitForPreviouslyUnrecognizedPosition
Tax Year 2007
Income Tax Contingency [Line Items]
Proposed deduction disallowance by tax authorities			130.0biib_ProposedDisallowanceByTaxAuthoritiesForPaymentForServices / us-gaap_TaxPeriodAxis = us-gaap_TaxYear2007Member
Tax Year 2008
Income Tax Contingency [Line Items]
Proposed deduction disallowance by tax authorities			130.0biib_ProposedDisallowanceByTaxAuthoritiesForPaymentForServices / us-gaap_TaxPeriodAxis = us-gaap_TaxYear2008Member
Tax Year 2009
Income Tax Contingency [Line Items]
Proposed deduction disallowance by tax authorities			$ 130.0biib_ProposedDisallowanceByTaxAuthoritiesForPaymentForServices / us-gaap_TaxPeriodAxis = us-gaap_TaxYear2009Member

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Net benefit for a previously unrecognized position

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Proposed disallowance by tax authorities for payment for services.

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- Definition

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to 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 740

-SubTopic 10

-Section 50

-Paragraph 13

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



Reference 3: 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=26873400&loc=d3e23780-122690



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

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- 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

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