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Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Marketable Debt and Equity Securities
Fair Value	$ 2,051.2us-gaap_AvailableForSaleSecurities	$ 2,111.1us-gaap_AvailableForSaleSecurities
Gross Unrealized Gains	1.9us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	0.7us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Gross Unrealized Losses	(0.8)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	(2.4)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Amortized Cost	2,050.1us-gaap_AvailableForSaleSecuritiesAmortizedCost	2,112.8us-gaap_AvailableForSaleSecuritiesAmortizedCost
Corporate debt securities Current
Marketable Debt and Equity Securities
Fair Value	385.3us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesCurrentMember	370.4us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesCurrentMember
Gross Unrealized Gains	0.1us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesCurrentMember	0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesCurrentMember
Gross Unrealized Losses	(0.1)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesCurrentMember	(0.2)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesCurrentMember
Amortized Cost	385.3us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesCurrentMember	370.6us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesCurrentMember
Corporate debt securities Non-current
Marketable Debt and Equity Securities
Fair Value	605.7us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesNonCurrentMember	692.6us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesNonCurrentMember
Gross Unrealized Gains	0.8us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesNonCurrentMember	0.2us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesNonCurrentMember
Gross Unrealized Losses	(0.4)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesNonCurrentMember	(1.5)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesNonCurrentMember
Amortized Cost	605.3us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesNonCurrentMember	693.9us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_CorporateDebtSecuritiesNonCurrentMember
Government securities Current
Marketable Debt and Equity Securities
Fair Value	288.5us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesCurrentMember	269.9us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesCurrentMember
Gross Unrealized Gains	0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesCurrentMember	0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesCurrentMember
Gross Unrealized Losses	(0.1)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesCurrentMember	(0.1)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesCurrentMember
Amortized Cost	288.6us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesCurrentMember	270.0us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesCurrentMember
Government securities Non-current
Marketable Debt and Equity Securities
Fair Value	562.1us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesNonCurrentMember	579.9us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesNonCurrentMember
Gross Unrealized Gains	0.6us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesNonCurrentMember	0.3us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesNonCurrentMember
Gross Unrealized Losses	(0.1)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesNonCurrentMember	(0.4)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesNonCurrentMember
Amortized Cost	561.6us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesNonCurrentMember	580.0us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_GovernmentSecuritiesNonCurrentMember
Mortgage and other asset backed securities Current
Marketable Debt and Equity Securities
Fair Value	0.1us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember	0.2us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember
Gross Unrealized Gains	0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember	0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember
Gross Unrealized Losses	0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember	0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember
Amortized Cost	0.1us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember	0.2us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember
Mortgage and other asset backed securities Non-current
Marketable Debt and Equity Securities
Fair Value	209.5us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember	198.1us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember
Gross Unrealized Gains	0.4us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember	0.2us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember
Gross Unrealized Losses	(0.1)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember	(0.2)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember
Amortized Cost	209.2us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember	198.1us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember
Marketable equity securities
Marketable Debt and Equity Securities
Fair Value	6.2us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MarketableEquitySecuritiesMember	6.9us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MarketableEquitySecuritiesMember
Gross Unrealized Gains	0.3us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MarketableEquitySecuritiesMember	1.2us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MarketableEquitySecuritiesMember
Gross Unrealized Losses	0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MarketableEquitySecuritiesMember	(0.2)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MarketableEquitySecuritiesMember
Amortized Cost	$ 5.9us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MarketableEquitySecuritiesMember	$ 5.9us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = biib_MarketableEquitySecuritiesMember

X

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

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-Publisher FASB

-Name Accounting Standards Codification

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-Section 50

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-Section 45

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ 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 210.5-02.2)

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