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Share-Based Payments (Details 1) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Summary of share based compensation expense associated with different programs
Subtotal	$ 91.0us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax	$ 76.6us-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	87.6biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses	74.0biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses
Market stock units
Summary of share based compensation expense associated with different programs
Share-based compensation expense	17.0us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = biib_MarketStockUnitsMember	14.9us-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	31.9us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = biib_TimeVestedRestrictedStockUnitsMember	29.2us-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	23.8us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = biib_CashSettledPerformanceSharesMember	21.4us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = biib_CashSettledPerformanceSharesMember
Performance units
Summary of share based compensation expense associated with different programs
Share-based compensation expense	13.2us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = us-gaap_PerformanceSharesMember	6.3us-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	$ 5.1us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = biib_EmployeeStockPurchasePlanMember	$ 4.8us-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.

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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



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-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

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-Section 50

-Paragraph 2

-Subparagraph (h)(1)(i)

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-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section F

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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.

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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

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-Publisher FASB

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-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(1)(ii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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