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Equity (Details) (USD $) | 3 Months Ended
--- | ---
Mar. 31, 2015 | Mar. 31, 2014
Reconciliation of equity attributable to non-controlling interests |  |
NCI, beginning of period | $ 5,030,000 us-gaap_MinorityInterest | $ 600,000 us-gaap_MinorityInterest
Net income (loss) attributable to NCI, net of tax | (2,367,000)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest | (228,000)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest
Fair value of net assets and liabilities acquired and assigned to NCI | 0 us-gaap_NoncontrollingInterestIncreaseFromBusinessCombination | 4,000,000 us-gaap_NoncontrollingInterestIncreaseFromBusinessCombination
Translation adjustment and other | 100,000 biib_Noncontrollinginterestincreasedecreaseother | 0 biib_Noncontrollinginterestincreasedecreaseother
NCI, end of period | $ 2,687,000 us-gaap_MinorityInterest | $ 4,400,000 us-gaap_MinorityInterest

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- Definition Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which is directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent (that is, noncontrolling interest, previously referred to as minority interest). + 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 210.5-02.31) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 27 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 20 -Article 7 + Details
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