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Acquisitions Acquisitions (Details 1) (USD $) | Mar. 31, 2015 | Dec. 31, 2014 | Feb. 12, 2015
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Business Acquisition [Line Items] |  |  |
Goodwill | $ 1,849,852,000 us-gaap_Goodwill | $ 1,760,249,000 us-gaap_Goodwill |
Convergence Pharmaceuticals |  |  |
Business Acquisition [Line Items] |  |  |
In-process research and development and other intangible assets |  |  | 424,600,000 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember
Other intangible assets |  |  | 7,600,000 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember
Goodwill |  |  | 92,300,000 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember
Deferred tax liability |  |  | (84,900,000)us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilitiesNoncurrent / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember
Other, net |  |  | (1,000,000)us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentAssets / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember
Total purchase price |  |  | $ 438,600,000 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = biib_ConvergencePharmaceuticalsMember

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- Definition Amount of deferred tax liability attributable to taxable temporary differences due after one year or the normal operating cycle, if longer, assumed 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 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 Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed. + 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 55 -Paragraph 37 -URI http://asc.fasb.org/extlink&oid=35744172&loc=d3e2207-128464 + Details
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