XML 89 R68.htm IDEA: XBRL DOCUMENT v2.4.1.9
Fair Value Measurements (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Assets:      
Marketable debt securities $ 2,111.1us-gaap_AvailableForSaleSecurities $ 1,245.9us-gaap_AvailableForSaleSecurities  
Fair Value, Measurements, Recurring      
Assets:      
Cash equivalents 716.3us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
424.7us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Derivative contracts 72.7us-gaap_ForeignCurrencyContractAssetFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
3.8us-gaap_ForeignCurrencyContractAssetFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Plan assets for deferred compensation 36.9biib_PlanAssetsForDeferredCompensationFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
22.7biib_PlanAssetsForDeferredCompensationFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Total 2,958.4us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
1,730.2us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Liabilities:      
Derivative contracts 5.4us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
23.5us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Contingent consideration obligations 215.5us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
280.9us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
293.9us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
Total 220.9us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
304.4us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Fair Value, Measurements, Recurring | Corporate debt securities      
Assets:      
Marketable debt securities 1,063.0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
439.8us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Fair Value, Measurements, Recurring | Government securities      
Assets:      
Marketable debt securities 849.8us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
674.7us-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 198.3us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
131.4us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Fair Value, Measurements, Recurring | Marketable equity securities      
Assets:      
Investments 6.9us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= biib_MarketableEquitySecuritiesMember
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
11.2us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= biib_MarketableEquitySecuritiesMember
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Fair Value, Measurements, Recurring | Venture capital investments      
Assets:      
Investments 14.5us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= biib_VentureCapitalInvestmentsMember
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
21.9us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= biib_VentureCapitalInvestmentsMember
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
20.3us-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 0us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Derivative contracts 0us-gaap_ForeignCurrencyContractAssetFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-gaap_ForeignCurrencyContractAssetFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Plan assets for deferred compensation 0biib_PlanAssetsForDeferredCompensationFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0biib_PlanAssetsForDeferredCompensationFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Total 6.9us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
11.2us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Liabilities:      
Derivative contracts 0us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Contingent consideration obligations 0us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Total 0us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-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 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-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 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-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 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-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 6.9us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= biib_MarketableEquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
11.2us-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 0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= biib_VentureCapitalInvestmentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-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 716.3us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
424.7us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Derivative contracts 72.7us-gaap_ForeignCurrencyContractAssetFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
3.8us-gaap_ForeignCurrencyContractAssetFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Plan assets for deferred compensation 36.9biib_PlanAssetsForDeferredCompensationFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
22.7biib_PlanAssetsForDeferredCompensationFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Total 2,937.0us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
1,697.1us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Liabilities:      
Derivative contracts 5.4us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
23.5us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Contingent consideration obligations 0us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Total 5.4us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
23.5us-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 1,063.0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
439.8us-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 849.8us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
674.7us-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 198.3us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
131.4us-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 0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= biib_MarketableEquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-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 0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= biib_VentureCapitalInvestmentsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-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 0us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Derivative contracts 0us-gaap_ForeignCurrencyContractAssetFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-gaap_ForeignCurrencyContractAssetFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Plan assets for deferred compensation 0biib_PlanAssetsForDeferredCompensationFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0biib_PlanAssetsForDeferredCompensationFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Total 14.5us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
21.9us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Liabilities:      
Derivative contracts 0us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
0us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Contingent consideration obligations 215.5us-gaap_BusinessCombinationContingentConsiderationLiability
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= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
280.9us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
 
Total 215.5us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
280.9us-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 0us-gaap_AvailableForSaleSecurities
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0us-gaap_AvailableForSaleSecurities
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Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Government securities      
Assets:      
Marketable debt securities 0us-gaap_AvailableForSaleSecurities
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0us-gaap_AvailableForSaleSecurities
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Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities      
Assets:      
Marketable debt securities 0us-gaap_AvailableForSaleSecurities
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0us-gaap_AvailableForSaleSecurities
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Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Marketable equity securities      
Assets:      
Investments 0us-gaap_InvestmentsFairValueDisclosure
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0us-gaap_InvestmentsFairValueDisclosure
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Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Venture capital investments      
Assets:      
Investments $ 14.5us-gaap_InvestmentsFairValueDisclosure
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$ 21.9us-gaap_InvestmentsFairValueDisclosure
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