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Acquisitions Acquisitions (Details Textual) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Feb. 12, 2015
Business Acquisition [Line Items] |  |  |
Contingent consideration | $ 238.5 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases | $ 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases |
Convergence Pharmaceuticals |  |  |
Business Acquisition [Line Items] |  |  |
Cash portion of consideration | 200.1 us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember |  |
Potential future milestone payments |  |  | 450.0 biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember
Contingent consideration | 238.5 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember |  |
Discount rate | 2.00%us-gaap_FairValueInputsDiscountRate / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember |  |
In-process research and development and other intangible assets |  |  | 424.6 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember
Convergence Pharmaceuticals | CNV1014802 |  |  |
Business Acquisition [Line Items] |  |  |
Potential future milestone payments |  |  | 350.0 biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember / us-gaap_ProductOrServiceAxis = biib_CNV1014802Member
Discount rate | 11.00%us-gaap_FairValueInputsDiscountRate / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember / us-gaap_ProductOrServiceAxis = biib_CNV1014802Member |  |
Remaining cost to complete CNV1014802 |  |  | 143.0 us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember / us-gaap_ProductOrServiceAxis = biib_CNV1014802Member
In-process research and development and other intangible assets |  |  | 170.0 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember / us-gaap_ProductOrServiceAxis = biib_CNV1014802Member
Convergence Pharmaceuticals | Neuropathic pain indications |  |  |
Business Acquisition [Line Items] |  |  |
Remaining cost to complete CNV1014802 |  |  | $ 446.0 us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember / us-gaap_ProductOrServiceAxis = biib_NeuropathicpainindicationsMember

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- Definition Potential future milestone payments commitment to third party approximately. + References No definition available. + Details
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- Details
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- Definition For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 + Details
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- Definition Amount of assets, excluding financial assets and goodwill, that lack physical substance, having a projected indefinite period of benefit, acquired at the acquisition date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472 + Details
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- Definition Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Amount of purchases of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585 + Details
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