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Share-Based Payments (Details Textual) (USD $)
In Millions, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Plan
Dec. 31, 2013
Dec. 31, 2012
Share-Based Payments (Textual)      
Windfall tax benefits from vesting of stock awards, exercises of stock options and ESPP participation $ 96.4biib_WindfallTaxBenefitsFromVestingOfStockAwardsExercisesOfStockOptionsAndEsppParticipation $ 73.5biib_WindfallTaxBenefitsFromVestingOfStockAwardsExercisesOfStockOptionsAndEsppParticipation $ 54.7biib_WindfallTaxBenefitsFromVestingOfStockAwardsExercisesOfStockOptionsAndEsppParticipation
Unrecognized compensation cost related to unvested share-based compensation 198.5us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized    
Weighted-average period to recognize the cost of unvested awards 1 year 11 months    
Number of share-based compensation plans pursuant to which awards are currently being made 3biib_NumberOfShareBasedCompensationPlansPursuantToWhichAwardsAreCurrentlyBeingMade    
Number of shares granted under stock options 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Total intrinsic value of options exercised 42.7us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue 86.2us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue 63.0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Aggregate intrinsic values of options outstanding 62.6us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue    
Weighted average remaining contractual term for options outstanding 3 years 0 months    
Market stock units      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of equal annual increments 3biib_NumberOfEqualAnnualIncrements
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember		 4biib_NumberOfEqualAnnualIncrements
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember		  
Percentage of earnings of the target number of units granted based on actual stock performance (Minimum) 0.00%biib_MinimumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember		 0.00%biib_MinimumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember		  
Percentage of earnings of the target number of units granted based on actual stock performance (Maximum) 200.00%biib_MaximumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember		 150.00%biib_MaximumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember		  
Number of shares granted 246,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember		 [1]    
Number of days for calculation of average closing stock price 30 days 60 days  
Total fair value of vested awards 117.4us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember		 50.9us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember		 29.6us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember
Weighted average grant date fair value $ 395.22us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember		 [1] $ 193.45us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember		  
Cash settled performance units      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of equal annual increments 3biib_NumberOfEqualAnnualIncrements
/ us-gaap_AwardTypeAxis
= biib_CashSettledPerformanceSharesMember		    
Percentage of earnings of the target number of units granted based on actual stock performance (Minimum) 0.00%biib_MinimumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent
/ us-gaap_AwardTypeAxis
= biib_CashSettledPerformanceSharesMember		    
Percentage of earnings of the target number of units granted based on actual stock performance (Maximum) 200.00%biib_MaximumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent
/ us-gaap_AwardTypeAxis
= biib_CashSettledPerformanceSharesMember		    
Number of shares granted 177,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= biib_CashSettledPerformanceSharesMember		 [2]    
Number of days for calculation of average closing stock price 30 days 60 days  
Cash in settlement of CSPS awards upon vesting 92.8us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards
/ us-gaap_AwardTypeAxis
= biib_CashSettledPerformanceSharesMember		 48.3us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards
/ us-gaap_AwardTypeAxis
= biib_CashSettledPerformanceSharesMember		 28.7us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards
/ us-gaap_AwardTypeAxis
= biib_CashSettledPerformanceSharesMember
Performance units      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of equal annual increments 3biib_NumberOfEqualAnnualIncrements
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		    
Percentage of earnings of the target number of units granted based on actual stock performance (Minimum) 0.00%biib_MinimumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		    
Percentage of earnings of the target number of units granted based on actual stock performance (Maximum) 200.00%biib_MaximumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		    
Number of shares granted 57,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 [2]    
Number of days for calculation of average closing stock price 30 days    
Time-vested restricted stock units      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of shares granted 464,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= biib_TimeVestedRestrictedStockUnitsMember		 [3]    
Total fair value of vested awards $ 281.1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= biib_TimeVestedRestrictedStockUnitsMember		 $ 209.7us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= biib_TimeVestedRestrictedStockUnitsMember		 $ 191.9us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= biib_TimeVestedRestrictedStockUnitsMember
Weighted average grant date fair value $ 321.72us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= biib_TimeVestedRestrictedStockUnitsMember		 [3] $ 176.53us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= biib_TimeVestedRestrictedStockUnitsMember		 $ 124.54us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= biib_TimeVestedRestrictedStockUnitsMember
Directors Plan      
Share-based Compensation Arrangement by Share-based Payment Award      
Total number of shares of common stock for issuance 1,600,000biib_NumberOfSharesOfCommonStockForIssuance
/ biib_ShareBasedCompensationPlanAxis
= biib_DirectorsPlanMember		    
Ratio of total number of shares reserved under the plan 1.5-to-1    
Omnibus Plans      
Share-based Compensation Arrangement by Share-based Payment Award      
Total number of shares of common stock for issuance 15,000,000biib_NumberOfSharesOfCommonStockForIssuance
/ biib_ShareBasedCompensationPlanAxis
= biib_OmnibusPlansMember		    
Ratio of total number of shares reserved under the plan 1.5-to-1    
Attainment Of Performance Criteria in 2013 | Market stock units      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of shares granted 22,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember
/ biib_ShareBasedCompensationPlanAxis
= biib_AttainmentOfPerformanceCriteriain2013Member		    
Attainment Of Performance Criteria in 2013 | Cash settled performance units      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of shares granted 106,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= biib_CashSettledPerformanceSharesMember
/ biib_ShareBasedCompensationPlanAxis
= biib_AttainmentOfPerformanceCriteriain2013Member		    
Attainment Of Performance Criteria in 2012 | Market stock units      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of shares granted 27,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember
/ biib_ShareBasedCompensationPlanAxis
= biib_AttainmentOfPerformanceCriteriain2012Member		    
Attainment Of Performance Criteria in 2011 | Market stock units      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of shares granted 37,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember
/ biib_ShareBasedCompensationPlanAxis
= biib_AttainmentOfPerformanceCriteriain2011Member		    
Attainment Of Performance Criteria in 2010 | Market stock units      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of shares granted 33,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember
/ biib_ShareBasedCompensationPlanAxis
= biib_AttainmentOfPerformanceCriteriain2010Member		    
Annual Vesting | Time-vested restricted stock units      
Share-based Compensation Arrangement by Share-based Payment Award      
Time vested RSU vesting percentage 33.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= biib_TimeVestedRestrictedStockUnitsMember
/ us-gaap_VestingAxis
= biib_AnnualVestingMember		    
Director [Member] | Time-vested restricted stock units      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of shares granted 9,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= biib_TimeVestedRestrictedStockUnitsMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember		    
[1] MSUs granted in 2014 include approximately 22,000, 27,000, 37,000 and 33,000 MSUs issued in 2014 based upon the attainment of performance criteria set for 2013, 2012, 2011 and 2010, respectively, in relation to awards granted in those years. The remainder of MSUs granted during 2014 include awards granted in conjunction with our annual awards made in February 2014 and MSUs granted in conjunction with the hiring of employees. These grants reflect the target number of shares eligible to be earned at the time of grant.
[2] CSPUs granted in 2014 include approximately 106,000 CSPUs issued in 2014 based upon the attainment of performance criteria set for 2013 in relation to awards granted in 2013. The remainder of the CSPUs granted in 2014 include awards granted in conjunction with our annual awards made in February 2014 and CSPUs granted in conjunction with the hiring of employees. These grants reflect the target number of shares eligible to be earned at the time of grant.
[3] RSUs granted in 2014 primarily represent RSUs granted in conjunction with our annual awards made in February 2014 and awards made in conjunction with the hiring of new employees. RSUs granted in 2014 also include approximately 9,000 RSUs granted to our Board of Directors.