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Share-Based Payments (Details 1) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Summary of share based compensation expense associated with different programs |  |
Subtotal | $ 91.0 us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax | $ 76.6 us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax
Capitalized share-based compensation costs | (3.4)us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount | (2.6)us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
Share-based compensation expense included in total costs and expenses | 87.6 biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses | 74.0 biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses
Market stock units |  |
Summary of share based compensation expense associated with different programs |  |
Share-based compensation expense | 17.0 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = biib_MarketStockUnitsMember | 14.9 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = biib_MarketStockUnitsMember
Time-vested restricted stock units |  |
Summary of share based compensation expense associated with different programs |  |
Share-based compensation expense | 31.9 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = biib_TimeVestedRestrictedStockUnitsMember | 29.2 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = biib_TimeVestedRestrictedStockUnitsMember
Cash settled performance shares |  |
Summary of share based compensation expense associated with different programs |  |
Share-based compensation expense | 23.8 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = biib_CashSettledPerformanceSharesMember | 21.4 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = biib_CashSettledPerformanceSharesMember
Performance units |  |
Summary of share based compensation expense associated with different programs |  |
Share-based compensation expense | 13.2 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = us-gaap_PerformanceSharesMember | 6.3 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = us-gaap_PerformanceSharesMember
Employee stock purchase plan |  |
Summary of share based compensation expense associated with different programs |  |
Share-based compensation expense | $ 5.1 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = biib_EmployeeStockPurchasePlanMember | $ 4.8 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = biib_EmployeeStockPurchasePlanMember

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- Definition Share based compensation expense included in costs and expenses. + References No definition available. + Details
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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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- Definition The amount of expense, net of income tax, recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References No definition available. + Details
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- Definition Represents the compensation cost capitalized during the period arising from equity-based compensation arrangements (for example, shares of stock, units, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + 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 (h)(1)(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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