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Derivative Instruments (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Derivative Instruments (Textual) [Abstract]      
Lower range of durations of foreign currency forward contracts 1 month   1 month
Higher range of durations of foreign currency forward contracts 21 months   15 months
Gain/Loss on fair value of foreign currency forward contracts $ 159.8biib_UnrealizedGainLossesOnForeignCurrencyDerivativesNetBeforeTax   $ 72.1biib_UnrealizedGainLossesOnForeignCurrencyDerivativesNetBeforeTax
Expected settlement time for contracts, in months 21 months    
Net gains (losses) of other income (expense) related to foreign currency forward contracts 9.7us-gaap_GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments 1.4us-gaap_GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments  
Designated as hedging instrument      
Derivative Instruments (Textual) [Abstract]      
Aggregate notional amount 1,588.5invest_DerivativeNotionalAmount
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
  1,302.3invest_DerivativeNotionalAmount
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
Not designated as hedging instrument      
Derivative Instruments (Textual) [Abstract]      
Aggregate notional amount 494.2invest_DerivativeNotionalAmount
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
  365.2invest_DerivativeNotionalAmount
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Foreign Exchange Contract | Other current assets | Designated as hedging instrument      
Summary of Derivatives designated as Hedging Instruments      
Derivative asset, fair value, net 160.1us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
  69.5us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
Foreign Exchange Contract | Other current assets | Not designated as hedging instrument      
Summary of Derivatives designated as Hedging Instruments      
Derivative asset, fair value, net 1.9us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
  1.3us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Foreign Exchange Contract | Investments and other assets | Designated as hedging instrument      
Summary of Derivatives designated as Hedging Instruments      
Derivative asset, fair value, net 6.2us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
  1.9us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
Foreign Exchange Contract | Accrued expenses and other | Designated as hedging instrument      
Summary of Derivatives designated as Hedging Instruments      
Derivative liability, fair value, net 1.3us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_AccruedLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
   
Foreign Exchange Contract | Accrued expenses and other | Not designated as hedging instrument      
Summary of Derivatives designated as Hedging Instruments      
Derivative liability, fair value, net 7.6us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_AccruedLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
  5.4us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_AccruedLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Foreign Exchange Contract | Other long-term liabilities | Designated as hedging instrument      
Summary of Derivatives designated as Hedging Instruments      
Derivative liability, fair value, net 3.7us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherNoncurrentLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
   
Revenue | Foreign Exchange Contract      
Derivative Instruments (Textual) [Abstract]      
Net gains (losses) in product revenue for the settlement of certain effective cash flow hedge instruments 35.0us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember
(4.7)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember
 
Other income (expense) | Foreign Exchange Contract      
Derivative Instruments (Textual) [Abstract]      
Net gains (losses) in earnings of Foreign currency forward contracts due to hedge ineffectiveness 2.2us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherNonoperatingIncomeExpenseMember
(0.2)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherNonoperatingIncomeExpenseMember
 
Euro | Designated as hedging instrument      
Derivative Instruments (Textual) [Abstract]      
Aggregate notional amount 1,482.3invest_DerivativeNotionalAmount
/ us-gaap_CurrencyAxis
= currency_EUR
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
  1,174.6invest_DerivativeNotionalAmount
/ us-gaap_CurrencyAxis
= currency_EUR
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
Canadian Dollar | Designated as hedging instrument      
Derivative Instruments (Textual) [Abstract]      
Aggregate notional amount 45.3invest_DerivativeNotionalAmount
/ us-gaap_CurrencyAxis
= currency_CAD
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
  56.7invest_DerivativeNotionalAmount
/ us-gaap_CurrencyAxis
= currency_CAD
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
British Pound Sterling | Designated as hedging instrument      
Derivative Instruments (Textual) [Abstract]      
Aggregate notional amount 29.2invest_DerivativeNotionalAmount
/ us-gaap_CurrencyAxis
= currency_GBP
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
  34.5invest_DerivativeNotionalAmount
/ us-gaap_CurrencyAxis
= currency_GBP
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
Japanese Yen | Designated as hedging instrument      
Derivative Instruments (Textual) [Abstract]      
Aggregate notional amount 16.6invest_DerivativeNotionalAmount
/ us-gaap_CurrencyAxis
= currency_JPY
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
  16.6invest_DerivativeNotionalAmount
/ us-gaap_CurrencyAxis
= currency_JPY
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
Australian Dollar | Designated as hedging instrument      
Derivative Instruments (Textual) [Abstract]      
Aggregate notional amount $ 15.1invest_DerivativeNotionalAmount
/ us-gaap_CurrencyAxis
= currency_AUD
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
  $ 19.9invest_DerivativeNotionalAmount
/ us-gaap_CurrencyAxis
= currency_AUD
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember