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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 | $ 159.8 biib_UnrealizedGainLossesOnForeignCurrencyDerivativesNetBeforeTax |  | $ 72.1 biib_UnrealizedGainLossesOnForeignCurrencyDerivativesNetBeforeTax
Expected settlement time for contracts, in months | 21 months |  |
Net gains (losses) of other income (expense) related to foreign currency forward contracts | 9.7 us-gaap_GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments | 1.4 us-gaap_GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments |
Designated as hedging instrument |  |  |
Derivative Instruments (Textual) [Abstract] |  |  |
Aggregate notional amount | 1,588.5 invest_DerivativeNotionalAmount / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  | 1,302.3 invest_DerivativeNotionalAmount / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember
Not designated as hedging instrument |  |  |
Derivative Instruments (Textual) [Abstract] |  |  |
Aggregate notional amount | 494.2 invest_DerivativeNotionalAmount / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 365.2 invest_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 | 160.1 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  | 69.5 us-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 | 1.9 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 1.3 us-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 | 6.2 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherInvestmentsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  | 1.9 us-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 | 1.3 us-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 | 7.6 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_AccruedLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 5.4 us-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 | 3.7 us-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 | 35.0 us-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 | 2.2 us-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 | 1,482.3 invest_DerivativeNotionalAmount / us-gaap_CurrencyAxis = currency_EUR / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  | 1,174.6 invest_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 | 45.3 invest_DerivativeNotionalAmount / us-gaap_CurrencyAxis = currency_CAD / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  | 56.7 invest_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 | 29.2 invest_DerivativeNotionalAmount / us-gaap_CurrencyAxis = currency_GBP / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  | 34.5 invest_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 | 16.6 invest_DerivativeNotionalAmount / us-gaap_CurrencyAxis = currency_JPY / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  | 16.6 invest_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 | $ 15.1 invest_DerivativeNotionalAmount / us-gaap_CurrencyAxis = currency_AUD / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  | $ 19.9 invest_DerivativeNotionalAmount / us-gaap_CurrencyAxis = currency_AUD / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember

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- Definition Amount of net unrealized gain (loss) related to the change in fair value of foreign currency exchange rate derivatives designated as cash flow hedging instruments. Recorded in accumulated other comprehensive income to the extent that the cash flow hedge is determined to be effective. + References No definition available. + Details
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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226000-175313 + Details
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- Definition Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226000-175313 + Details
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- Definition High end period remaining until maturity for the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4D -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624177-113959 + Details
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- Definition The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 25 -Section 50 -Paragraph 1 -Subparagraph (a)(1),(2) -URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986 + Details
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- Definition Low end period remaining until maturity of derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Amount of gain (loss) recognized in earnings in the period from the increase (decrease) in fair value of foreign currency derivatives not designated as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 + Details
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- Definition Maximum amount of time remaining before foreign currency exchange rate derivatives mature or expire, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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