XML 86 R54.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014 | Mar. 31, 2014 | Dec. 31, 2013
--- | --- | --- | --- | ---
Assets: |  |  |  |
Marketable debt securities | $ 2,051.2 us-gaap_AvailableForSaleSecurities | $ 2,111.1 us-gaap_AvailableForSaleSecurities |  |
Fair Value, Measurements, Recurring |  |  |  |
Assets: |  |  |  |
Cash equivalents | 752.5 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 716.3 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Derivative contracts | 168.2 us-gaap_ForeignCurrencyContractAssetFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 72.7 us-gaap_ForeignCurrencyContractAssetFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Plan assets for deferred compensation | 44.1 biib_PlanAssetsForDeferredCompensationFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 36.9 biib_PlanAssetsForDeferredCompensationFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Total | 3,034.3 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 2,958.4 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Liabilities: |  |  |  |
Derivative contracts | 12.6 us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 5.4 us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Contingent consideration obligations | 461.8 us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 215.5 us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 275.1 us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 280.9 us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember
Total | 474.4 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 220.9 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Fair Value, Measurements, Recurring | Corporate debt securities |  |  |  |
Assets: |  |  |  |
Marketable debt securities | 991.0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 1,063.0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Fair Value, Measurements, Recurring | Government securities |  |  |  |
Assets: |  |  |  |
Marketable debt securities | 850.6 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 849.8 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities |  |  |  |
Assets: |  |  |  |
Marketable debt securities | 209.6 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 198.3 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Fair Value, Measurements, Recurring | Marketable equity securities |  |  |  |
Assets: |  |  |  |
Investments | 6.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_MarketableEquitySecuritiesMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 6.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_MarketableEquitySecuritiesMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Fair Value, Measurements, Recurring | Venture capital investments |  |  |  |
Assets: |  |  |  |
Investments | 12.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_VentureCapitalInvestmentsMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 14.5 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_VentureCapitalInvestmentsMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 23.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_VentureCapitalInvestmentsMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 21.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_VentureCapitalInvestmentsMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring |  |  |  |
Assets: |  |  |  |
Cash equivalents | 0 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Derivative contracts | 0 us-gaap_ForeignCurrencyContractAssetFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_ForeignCurrencyContractAssetFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Plan assets for deferred compensation | 0 biib_PlanAssetsForDeferredCompensationFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 biib_PlanAssetsForDeferredCompensationFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Total | 6.2 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 6.9 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Liabilities: |  |  |  |
Derivative contracts | 0 us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Contingent consideration obligations | 0 us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Total | 0 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Corporate debt securities |  |  |  |
Assets: |  |  |  |
Marketable debt securities | 0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Government securities |  |  |  |
Assets: |  |  |  |
Marketable debt securities | 0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities |  |  |  |
Assets: |  |  |  |
Marketable debt securities | 0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Marketable equity securities |  |  |  |
Assets: |  |  |  |
Investments | 6.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_MarketableEquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 6.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_MarketableEquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring | Venture capital investments |  |  |  |
Assets: |  |  |  |
Investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_VentureCapitalInvestmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_VentureCapitalInvestmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |  |  |
Assets: |  |  |  |
Cash equivalents | 752.5 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 716.3 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Derivative contracts | 168.2 us-gaap_ForeignCurrencyContractAssetFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 72.7 us-gaap_ForeignCurrencyContractAssetFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Plan assets for deferred compensation | 44.1 biib_PlanAssetsForDeferredCompensationFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 36.9 biib_PlanAssetsForDeferredCompensationFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Total | 3,016.0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 2,937.0 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Liabilities: |  |  |  |
Derivative contracts | 12.6 us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 5.4 us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Contingent consideration obligations | 0 us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Total | 12.6 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 5.4 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities |  |  |  |
Assets: |  |  |  |
Marketable debt securities | 991.0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 1,063.0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities |  |  |  |
Assets: |  |  |  |
Marketable debt securities | 850.6 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 849.8 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities |  |  |  |
Assets: |  |  |  |
Marketable debt securities | 209.6 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 198.3 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Marketable equity securities |  |  |  |
Assets: |  |  |  |
Investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_MarketableEquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_MarketableEquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Venture capital investments |  |  |  |
Assets: |  |  |  |
Investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_VentureCapitalInvestmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_VentureCapitalInvestmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |  |  |
Assets: |  |  |  |
Cash equivalents | 0 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Derivative contracts | 0 us-gaap_ForeignCurrencyContractAssetFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_ForeignCurrencyContractAssetFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Plan assets for deferred compensation | 0 biib_PlanAssetsForDeferredCompensationFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 biib_PlanAssetsForDeferredCompensationFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Total | 12.1 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 14.5 us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Liabilities: |  |  |  |
Derivative contracts | 0 us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Contingent consideration obligations | 461.8 us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 215.5 us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Total | 461.8 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 215.5 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Corporate debt securities |  |  |  |
Assets: |  |  |  |
Marketable debt securities | 0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Government securities |  |  |  |
Assets: |  |  |  |
Marketable debt securities | 0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities |  |  |  |
Assets: |  |  |  |
Marketable debt securities | 0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByAssetClassAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Marketable equity securities |  |  |  |
Assets: |  |  |  |
Investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_MarketableEquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_MarketableEquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Venture capital investments |  |  |  |
Assets: |  |  |  |
Investments | $ 12.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_VentureCapitalInvestmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | $ 14.5 us-gaap_InvestmentsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = biib_VentureCapitalInvestmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember |  |

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 + Details
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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 + Details
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- Definition Fair value of financial and nonfinancial obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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