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Fair Value Measurements (Details Textual) (USD $) In Millions, unless otherwise specified | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014 | Feb. 12, 2015
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |
Asset impairment charges | $ 0 us-gaap_AssetImpairmentCharges |  |  |
Changes in valuation techniques | 0 |  |  |
Contingent consideration obligations | 397.5 us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent |  | 200.0 us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent |
Contingent consideration | 238.5 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases |  |
6.875% Senior Notes due 2018 |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |
Stated rate on debt | 6.875%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember |  |  |
Convergence Pharmaceuticals |  |  |  |
Business Acquisition, Contingent Consideration [Line Items] |  |  |  |
Contingent consideration | 238.5 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember |  |  |
Potential future milestone payments |  |  |  | 450.0 biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember
Discount rate | 2.00%us-gaap_FairValueInputsDiscountRate / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember |  |  |
In-process research and development and other intangible assets |  |  |  | $ 424.6 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember

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