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Income Taxes (Details Textual) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2011
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Tax Credit Carryforward [Line Items]        
Foreign   $ (6,300,000)us-gaap_DeferredForeignIncomeTaxExpenseBenefit $ (8,600,000)us-gaap_DeferredForeignIncomeTaxExpenseBenefit $ 29,100,000us-gaap_DeferredForeignIncomeTaxExpenseBenefit
Tax accounting correction       29,000,000biib_TaxAccountingCorrection
Total deferred charges and prepaid taxes   238,900,000biib_Totaldeferredchargesandprepaidtaxes 248,900,000biib_Totaldeferredchargesandprepaidtaxes  
Deferred tax liability recognized     203,700,000biib_DeferredTaxLiabilityRecognized  
Net book value of deferred charge   539,800,000us-gaap_DeferredTaxLiabilities 691,500,000us-gaap_DeferredTaxLiabilities  
Net benefit for a previously unrecognized position     49,800,000biib_NetBenefitForPreviouslyUnrecognizedPosition  
Expense for non-income based state taxes     11,300,000biib_ExpenseForNonIncomeBasedStateTaxes  
Proposed Disallowance By Tax Authorities For Payment For Services 130,000,000biib_ProposedDisallowanceByTaxAuthoritiesForPaymentForServices      
Net effect of adjustments to our uncertain tax positions   2,000,000us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions    
Income Taxes (Textual)        
Undistributed Foreign Earnings Of Non Us Subsidiaries   4,600,000,000us-gaap_UndistributedEarningsOfForeignSubsidiaries    
Unrecognized tax benefit   53,600,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 32,500,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 109,500,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Net interest expense   4,100,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense 4,500,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense 100,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense
Unrecognized Tax Benefits, Interest on Income Taxes Accrued   17,600,000us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued 11,300,000us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued  
Notice of assessment of corporate withholding tax including penalties and interest   51,100,000us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued    
Minimum        
Tax Credit Carryforward [Line Items]        
Estimated Tax Liability On Undistributed Earnings Of Foreign Subsidiaries   1,500,000,000biib_EstimatedTaxLiabilityOnUndistributedEarningsOfForeignSubsidiaries
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Maximum        
Tax Credit Carryforward [Line Items]        
Estimated Tax Liability On Undistributed Earnings Of Foreign Subsidiaries   1,600,000,000biib_EstimatedTaxLiabilityOnUndistributedEarningsOfForeignSubsidiaries
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		    
General Business | Domestic Country        
Tax Credit Carryforward [Line Items]        
Operating Loss Carryforwards   28,500,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
/ us-gaap_TaxCreditCarryforwardAxis
= us-gaap_GeneralBusinessMember		    
Tax Credit Carryforward, Amount   6,600,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
/ us-gaap_TaxCreditCarryforwardAxis
= us-gaap_GeneralBusinessMember		    
General Business | State and Local Jurisdiction        
Tax Credit Carryforward [Line Items]        
Deferred Tax Assets, Operating Loss Carryforwards, State and Local   108,100,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
/ us-gaap_TaxCreditCarryforwardAxis
= us-gaap_GeneralBusinessMember		    
General Business | Foreign Tax Authority [Member]        
Tax Credit Carryforward [Line Items]        
Operating Loss Carryforwards   29,400,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember
/ us-gaap_TaxCreditCarryforwardAxis
= us-gaap_GeneralBusinessMember		    
Research | State and Local Jurisdiction        
Tax Credit Carryforward [Line Items]        
Tax Credit Carryforward, Amount   116,800,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
/ us-gaap_TaxCreditCarryforwardAxis
= us-gaap_ResearchMember		    
Hillerod Denmark Facility        
Tax Credit Carryforward [Line Items]        
Foreign       33,100,000us-gaap_DeferredForeignIncomeTaxExpenseBenefit
/ biib_FacilityLocationAxis
= biib_HillerodDenmarkFacilityMember
ZINBRYTA        
Tax Credit Carryforward [Line Items]        
Net book value of deferred charge   $ 179,900,000us-gaap_DeferredTaxLiabilities
/ us-gaap_ProductOrServiceAxis
= biib_ZINBRYTAMember		 $ 193,500,000us-gaap_DeferredTaxLiabilities
/ us-gaap_ProductOrServiceAxis
= biib_ZINBRYTAMember