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Fair Value Measurements (Details 1) (USD $) In Millions, unless otherwise specified | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014 | Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Unrealized gains included in earnings | $ 0 biib_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainIncludedInEarnings | $ 2.9 biib_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainIncludedInEarnings |  |
Unrealized losses included in earnings | (2.4)biib_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLossIncludedInEarnings | (1.2)biib_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLossIncludedInEarnings |  |
Fair Value, Measurements, Recurring | Venture capital investments |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Fair value, beginning of period |  |  | 14.5 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
Fair value, end of period | $ 12.1 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 | $ 14.5 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

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- Definition Fair value measurement with unobservable inputs reconciliation recurring basis asset gain included in earnings. + References No definition available. + Details
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- Definition Fair value measurement with unobservable inputs reconciliation recurring basis asset loss included in earnings. + References No definition available. + 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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