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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 | $ 45.6 us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued |  |
Net benefit for a previously unrecognized position | 16.4 biib_NetBenefitForPreviouslyUnrecognizedPosition |  |
Tax Year 2007 |  |  |
Income Tax Contingency [Line Items] |  |  |
Proposed deduction disallowance by tax authorities |  |  | 130.0 biib_ProposedDisallowanceByTaxAuthoritiesForPaymentForServices / us-gaap_TaxPeriodAxis = us-gaap_TaxYear2007Member
Tax Year 2008 |  |  |
Income Tax Contingency [Line Items] |  |  |
Proposed deduction disallowance by tax authorities |  |  | 130.0 biib_ProposedDisallowanceByTaxAuthoritiesForPaymentForServices / us-gaap_TaxPeriodAxis = us-gaap_TaxYear2008Member
Tax Year 2009 |  |  |
Income Tax Contingency [Line Items] |  |  |
Proposed deduction disallowance by tax authorities |  |  | $ 130.0 biib_ProposedDisallowanceByTaxAuthoritiesForPaymentForServices / us-gaap_TaxPeriodAxis = us-gaap_TaxYear2009Member

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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 + Details
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