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Commitments and Contingences (Details) (Fumapharm AG, USD $) | 12 Months Ended | 3 Months Ended |
--- | --- | --- | ---
Dec. 31, 2006 | Mar. 31, 2015 | Jun. 30, 2013
Business Acquisition, Contingent Consideration [Line Items] |  |  |
Cash portion of consideration | $ 220,000,000 us-gaap_PaymentsToAcquireBusinessesGross |  |
TECFIDERA |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |
Maximum contingent consideration in the form of development and approval milestones |  |  | 15,000,000 us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh / us-gaap_BusinessAcquisitionAxis = biib_FumapharmAgMember / us-gaap_ProductOrServiceAxis = biib_TecfideraMember
Four billion |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |
Cash portion of consideration |  | 250,000,000 us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = biib_FumapharmAgMember / biib_CumulativeSalesLevelAxis = biib_FourbillionMember |
Cumulative sales level |  | 4,000,000,000 biib_CumulativeSalesLevel / us-gaap_BusinessAcquisitionAxis = biib_FumapharmAgMember / biib_CumulativeSalesLevelAxis = biib_FourbillionMember |
Each additional one billion up to twenty billion |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |
Cumulative sales level |  | 20,000,000,000 biib_CumulativeSalesLevel / us-gaap_BusinessAcquisitionAxis = biib_FumapharmAgMember / biib_CumulativeSalesLevelAxis = biib_EachAdditionalOneBillionUpToTwentyBillionMember |
Payment of contingent payment for acquisition |  | 300,000,000 biib_MilestonePaymentDueOnPrior12MonthSales / us-gaap_BusinessAcquisitionAxis = biib_FumapharmAgMember / biib_CumulativeSalesLevelAxis = biib_EachAdditionalOneBillionUpToTwentyBillionMember |
One billion |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |
Cumulative sales level |  | 1,000,000,000 biib_CumulativeSalesLevel / us-gaap_BusinessAcquisitionAxis = biib_FumapharmAgMember / biib_CumulativeSalesLevelAxis = biib_OneBillionMember |
Three billion |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |
Cumulative sales level |  | $ 3,000,000,000 biib_CumulativeSalesLevel / us-gaap_BusinessAcquisitionAxis = biib_FumapharmAgMember / biib_CumulativeSalesLevelAxis = biib_ThreeBillionMember |

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- Definition Cumulative sales level + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Milestone payment due on the prior 12 month sales + References No definition available. + Details
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Namespace Prefix: | biib_
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Balance Type: | debit
Period Type: | instant

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- Details
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Balance Type: | na
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- Definition For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 + Details
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Period Type: | instant

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- Definition The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585 + Details
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- Details
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Namespace Prefix: |
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Period Type: |

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- Details
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- Details
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- Details
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