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Share-Based Payments (Details 5) (Market stock units, USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Assumptions used in valuation of market based stock units    
Expected dividend yield 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
Minimum range of risk-free interest rates 0.10%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum 0.10%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum
Maximum range of risk-free interest rates 0.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum 0.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum
Average stock price on grant date minimum $ 280.88biib_AverageStockPriceOnGrantDateInDollarsMinimum $ 150.33biib_AverageStockPriceOnGrantDateInDollarsMinimum
Average stock price on grant date maximum $ 335.65biib_AverageStockPriceOnGrantDateInDollarsMaximum $ 240.14biib_AverageStockPriceOnGrantDateInDollarsMaximum
Weighted average grant date fair value $ 395.22us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue [1] $ 193.45us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Minimum
    
Assumptions used in valuation of market based stock units    
Range of expected stock price volatility 31.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 21.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Maximum
    
Assumptions used in valuation of market based stock units    
Range of expected stock price volatility 35.10%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 25.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= biib_MarketStockUnitsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
[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.