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Financial Instruments Financial Instruments (Details Textual 2) (Strategic Investments, USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Strategic Investments |  |
Business Acquisition [Line Items] |  |
Strategic investment portfolio | $ 45.3 biib_StrategicInvestmentPortfolio / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_StrategicInvestmentsMember | $ 47.8 biib_StrategicInvestmentPortfolio / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_StrategicInvestmentsMember
Venture capital investments | 18.3 biib_VentureCapitalInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_StrategicInvestmentsMember | 21.4 biib_VentureCapitalInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_StrategicInvestmentsMember
Cost basis of equity securities of certain privately-owned companies and venture capital funds | $ 27.0 us-gaap_CostMethodInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_StrategicInvestmentsMember | $ 26.4 us-gaap_CostMethodInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_StrategicInvestmentsMember

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- Definition Strategic investment portfolio. + References No definition available. + Details
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- Definition Venture capital investments. + References No definition available. + Details
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- Details
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- Definition Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 325 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6872867&loc=d3e40691-111596 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 325 -SubTopic 20 -Section 35 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=29638308&loc=d3e40346-111594 + Details
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