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Financial Instruments (Details Textual 2) (Strategic Investments, USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Strategic Investments
   
Business Acquisition    
Strategic investment portfolio $ 47.8biib_StrategicInvestmentPortfolio
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$ 56.9biib_StrategicInvestmentPortfolio
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Venture capital investments 21.4biib_VentureCapitalInvestments
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33.1biib_VentureCapitalInvestments
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Cost basis of equity securities of certain privately-held companies and venture capital funds $ 26.4us-gaap_CostMethodInvestments
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$ 23.8us-gaap_CostMethodInvestments
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