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Income Taxes (Details)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Reconciliation between the U.S. federal statutory tax rate and effective tax rate
Statutory rate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
State taxes	(0.40%)us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	1.20%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Taxes on foreign earnings	(8.40%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential	(8.40%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential
Credits and net operating loss utilization	0.70%us-gaap_EffectiveIncomeTaxRateReconciliationTaxCredits	1.30%us-gaap_EffectiveIncomeTaxRateReconciliationTaxCredits
Purchased intangible assets	1.00%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciationAndAmortization	1.50%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciationAndAmortization
Manufacturing deduction	(1.60%)biib_EffectiveIncomeTaxRateReconciliationManufacturingDeduction	(2.00%)biib_EffectiveIncomeTaxRateReconciliationManufacturingDeduction
Permanent items	0.60%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther	0.30%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther
Other	0.10%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments	0.50%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Effective tax rate	25.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations	26.80%us-gaap_EffectiveIncomeTaxRateContinuingOperations

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Effective income tax rate reconciliation manufacturing deduction

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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-Name Accounting Standards Codification

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to statutory income tax expense (benefit) outside of the country of domicile.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to tax credits. Including, but not limited to, research credit, foreign tax credit, investment tax credit, and other tax credits.

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