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Investments in Variable Interest Entities (Details) (USD $) | 3 Months Ended |
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Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Investment in Variable Interest Entities (Textual) [Abstract] |  |  |
Research and development | $ 460,549,000 us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost | $ 528,884,000 us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost |
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities | 8,500,000 biib_InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities |  | 7,900,000 biib_InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities
Neurimmune |  |  |
Investment in Variable Interest Entities (Textual) [Abstract] |  |  |
Remaining potential development milestone payments and royalties on commercial sales under the terms of collaboration agreement | 335,000,000 biib_RemainingPotentialDevelopmentMilestonePaymentsAndRoyaltiesOnCommercialSalesUnderTermsOfCollaborationAgreement / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = biib_NeurimmuneMember |  |
Expected milestone payment for initiation of a late stage clinical trial | 60,000,000 biib_Expectedmilestonepaymentforinitiationofalatestageclinicaltrial / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = biib_NeurimmuneMember |  |
Research and development | $ 12,800,000 us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = biib_NeurimmuneMember | $ 11,000,000 us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = biib_NeurimmuneMember |

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- Definition Expected milestone payment for initiation of a late stage clinical trial + References No definition available. + Details
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- Definition Investment in biotechnology companies that are determined to be unconsolidated variable interest entities. + References No definition available. + Details
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- Details
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- Definition Expected total payments including upfront and milestone payments as well as the greater of an annual maintenance fee or usage-based royalty payments under the terms of collaboration agreement. + References No definition available. + Details
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- Definition The costs incurred in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, excluding in-process research and development acquired in a business combination consummated during the period. Excludes software research and development, which has a separate concept. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 730 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Research and Development -URI http://asc.fasb.org/extlink&oid=6523717 + Details
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- Details
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