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Share-Based Payments (Details 2) | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Market stock units |  |
Schedule of stock option grants |  |
Stock units | 175,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = biib_MarketStockUnitsMember | 214,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = biib_MarketStockUnitsMember
Cash settled performance shares |  |
Schedule of stock option grants |  |
Stock units | 112,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = biib_CashSettledPerformanceSharesMember | 172,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = biib_CashSettledPerformanceSharesMember
Performance units |  |
Schedule of stock option grants |  |
Stock units | 89,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = us-gaap_PerformanceSharesMember | 50,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = us-gaap_PerformanceSharesMember
Time-vested restricted stock units |  |
Schedule of stock option grants |  |
Stock units | 351,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = biib_TimeVestedRestrictedStockUnitsMember | 392,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = biib_TimeVestedRestrictedStockUnitsMember

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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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