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

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- 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) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 + Details
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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) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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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) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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