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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
Strategic Investments
Business Acquisition [Line Items]
Strategic investment portfolio	$ 45.3biib_StrategicInvestmentPortfolio / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_StrategicInvestmentsMember	$ 47.8biib_StrategicInvestmentPortfolio / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_StrategicInvestmentsMember
Venture capital investments	18.3biib_VentureCapitalInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_StrategicInvestmentsMember	21.4biib_VentureCapitalInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_StrategicInvestmentsMember
Cost basis of equity securities of certain privately-owned companies and venture capital funds	$ 27.0us-gaap_CostMethodInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_StrategicInvestmentsMember	$ 26.4us-gaap_CostMethodInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_StrategicInvestmentsMember

X

- Definition

Strategic investment portfolio.

+ References

No definition available.

+ Details

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X

- Definition

Venture capital investments.

+ References

No definition available.

+ Details

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- Details

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X

- 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

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