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Intangible Assets and Goodwill (Details) (USD $) | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014 | Feb. 12, 2015
Intangible assets |  |  |  |
Total intangible assets, gross | $ 7,627,600,000 us-gaap_IntangibleAssetsGrossExcludingGoodwill |  | $ 7,207,100,000 us-gaap_IntangibleAssetsGrossExcludingGoodwill |
Accumulated Amortization | (3,274,500,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization |  | (3,178,600,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization |
Intangible assets, net | 4,353,123,000 us-gaap_IntangibleAssetsNetExcludingGoodwill |  | 4,028,507,000 us-gaap_IntangibleAssetsNetExcludingGoodwill |
Intangible Assets and Goodwill (Textual) [Abstract] |  |  |  |
Amortization of acquired intangible assets | 95,903,000 us-gaap_AmortizationOfIntangibleAssets | 143,258,000 us-gaap_AmortizationOfIntangibleAssets |  |
Accumulated impairment losses related to goodwill | 0 us-gaap_GoodwillImpairedAccumulatedImpairmentLoss |  |  |
Intangible Assets And Goodwill (Additional Textual) [Abstract] |  |  |  |
Expected future amortization expense, 2015 (remaining nine months) | 269,000,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear |  |  |
Expected future amortization expense, 2016 | 313,300,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths |  |  |
Expected future amortization expense, 2017 | 286,100,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo |  |  |
Expected future amortization expense, 2018 | 281,600,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree |  |  |
Expected future amortization expense, 2019 | 270,400,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour |  |  |
Expected future amortization expense, 2020 | 263,400,000 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive |  |  |
Total | 1,683,800,000 biib_Finitelivedintangibleassetsfutureamortization |  |  |
Out-licensed patents |  |  |  |
Intangible assets |  |  |  |
Cost | 543,300,000 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_OutLicensedPatentsMember |  | 543,300,000 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_OutLicensedPatentsMember |
Accumulated Amortization | (487,800,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_OutLicensedPatentsMember |  | (481,700,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_OutLicensedPatentsMember |
Net | 55,500,000 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_OutLicensedPatentsMember |  | 61,600,000 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_OutLicensedPatentsMember |
Impairment of out-licensed patent |  | 34,700,000 us-gaap_ImpairmentOfIntangibleAssetsFinitelived / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_OutLicensedPatentsMember |  |
Out-licensed patents | Minimum |  |  |  |
Intangible assets |  |  |  |
Estimated life, (in years) | 13 years |  |  |
Out-licensed patents | Maximum |  |  |  |
Intangible assets |  |  |  |
Estimated life, (in years) | 23 years |  |  |
Developed technology |  |  |  |
Intangible assets |  |  |  |
Cost | 3,005,300,000 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_DevelopedTechnologyRightsMember |  | 3,005,300,000 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_DevelopedTechnologyRightsMember |
Accumulated Amortization | (2,447,500,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_DevelopedTechnologyRightsMember |  | (2,396,800,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_DevelopedTechnologyRightsMember |
Net | 557,800,000 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_DevelopedTechnologyRightsMember |  | 608,500,000 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_DevelopedTechnologyRightsMember |
Developed technology | Minimum |  |  |  |
Intangible assets |  |  |  |
Estimated life, (in years) | 15 years |  |  |
Developed technology | Maximum |  |  |  |
Intangible assets |  |  |  |
Estimated life, (in years) | 23 years |  |  |
Developed technology | AVONEX |  |  |  |
Intangible assets |  |  |  |
Net | 548,700,000 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_DevelopedTechnologyRightsMember / us-gaap_ProductOrServiceAxis = biib_AVONEXMember |  |  |
Acquired and in-licensed rights and patents |  |  |  |
Intangible assets |  |  |  |
Cost | 3,288,900,000 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_InLicensedPatentsMember |  | 3,280,400,000 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_InLicensedPatentsMember |
Accumulated Amortization | (339,200,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_InLicensedPatentsMember |  | (300,100,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_InLicensedPatentsMember |
Net | 2,949,700,000 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_InLicensedPatentsMember |  | 2,980,300,000 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_InLicensedPatentsMember |
Acquired and in-licensed rights and patents | Minimum |  |  |  |
Intangible assets |  |  |  |
Estimated life, (in years) | 6 years |  |  |
Acquired and in-licensed rights and patents | Maximum |  |  |  |
Intangible assets |  |  |  |
Estimated life, (in years) | 17 years |  |  |
In-process research and development |  |  |  |
Intangible assets |  |  |  |
Indefinite lived intangible assets useful life | Indefinite until commercialization |  |  |
Cost and Net | 726,100,000 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_InProcessResearchAndDevelopmentMember |  | 314,100,000 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_InProcessResearchAndDevelopmentMember |
Accumulated Amortization | 0 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_InProcessResearchAndDevelopmentMember |  | 0 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_InProcessResearchAndDevelopmentMember |
Trademarks and trade names |  |  |  |
Intangible assets |  |  |  |
Indefinite lived intangible assets useful life | Indefinite |  |  |
Cost and Net | 64,000,000 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksAndTradeNamesMember |  | 64,000,000 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksAndTradeNamesMember |
Accumulated Amortization | 0 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksAndTradeNamesMember |  | 0 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksAndTradeNamesMember |
Convergence Pharmaceuticals |  |  |  |
Intangible assets |  |  |  |
In-process research and development and other intangible assets |  |  |  | 424,600,000 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember
TYSABRI product | Acquired and in-licensed rights and patents |  |  |  |
Intangible assets |  |  |  |
Net | $ 2,886,600,000 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_BusinessAcquisitionAxis = biib_TysabriProductMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = biib_InLicensedPatentsMember |  |  |

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